3.7 Answers to "What is colour?" page

The previous page presents an explanation of colour produced for school children in response to Alan Alda's 2014 Flame Challenge, but which may also be helpful to adults. My video attempts to explain in simple terms the role of both trichromatic input and opponent output in colour vision: Although the opponent model has been widely accepted for many decades alongside trichromacy, most popular explanations of colour vision still omit it completely.

After studying that page, and/or watching the video here, readers should be able to spot some problems with these two rather misleading YouTube videos, and may like to compare their conclusions with my notes on this page.

This Is Not Yellow (Michael Stevens, Vsauce, 2012)

"Real" yellow light = "light with a wavelength of about 570 mm" (onscreen text, 0:55).

Stevens' basic point that screen yellow is made from a mixture of red and green lights is of course correct, and judging by the comments was news to a remarkably large number of his viewers. However, his explanation perpetuates an incomplete and partly inaccurate picture of the nature of colour. Like many other accounts of colour vision, Stevens' presentation includes a version of the trichromatic theory of input by means of three cone types, but completely omits the opponent theory of output of yellow/blue and red/green colour signals in the brain, and instead speaks as if colours themselves reside in the wavelengths of the spectrum. Stevens' statement here that the wavelengths of the spectrum that appear yellow are "real" yellow, and his inference that yellow-looking light lacking these wavelengths is "fake" yellow (see below), reflect the assumption that the colour yellow 'exists' in these wavelengths.

"Here in this room, this lemon is "subtractively yellow". It absorbs all visible wavelengths of light except for yellow light, which it reflects onto my retina."

This second mistake is a commonly repeated falsehood regarding the reflectance of yellow objects, also found for example in Michael Wilcox's book Blue and Yellow Don't Make Green. A lemon actually reflects much of the red, orange, yellow and green light that falls on it, and most of its yellow colour comes from the additive mixture of the reddish and greenish wavelengths, like the yellow on a screen. The same is true of all bright yellow objects:

"But, the screen that you are using to watch this video doesn't produce yellow light at all; in fact, it can only produce red, blue, or green light." ... "Absolutely no yellow is coming off of your screen and falling onto your retina."

By "no yellow" Stevens means no light from the yellow band of the spectrum, which is not strictly correct, because the green phosphor is not monochromatic (below). It might also be pointed out that the colour yellow is seen in a large part of the spectrum beyond the pure yellow band, and that the colours of both the yellowish green phosphor and the orangeish red phosphor have a component of yellow. It's true however that the screen gives off considerably less light in the pure yellow band of the spectrum than the lemon.

"The really cool, but kind of disturbing, thing about this is that here in the room, I am actually seeing "real" yellow light- but you are seeing "fake" yellow".

Based on his mistakes about the wavelengths reflected by yellow objects and the external existence of yellow in the spectrum itself, Stevens assumes that being yellow is the property of reflecting or giving off light from only the pure yellow band of the spectrum, and describes the computer screen as "fake yellow" because it does not meet this criterion. But given that light we see as yellow can contain either a single band of the spectrum (spectral yellow) or mostly two bands (screen yellow), or more than half the wavelengths of the spectrum (the light reflected by all bright yellow objects including lemons), it is impossible to justify Stevens' criterion. Since most of the yellow light we see in nature is a mixture of red to green wavelengths, one could actually argue that this broadband yellow is "real "yellow, and that the single-wavelength yellow of the spectrum is "fake" yellow. However, instead of talking in terms of real and fake yellow, it makes far more sense to say that yellow is a mental perception, not a wavelength.

Accepting this viewpoint, the title sentence "This Is Not Yellow" is actually true of the screen (and also of the lemon) in the specific sense that the colour yellow does not exist outside the observer, in the same way as Newton said that the rays to speak properly are not coloured. However in our normal, anthropocentric manner of speaking we would say that both the screen (unless magnified) and the lemon are yellow, because they are perceived to be yellow. This choice of alternative semantics should not be confused with the actual question of whether colour itself does in fact reside in the mind (as the modern scientific explanation indicates) or externally in the world (as some academic philosophers still maintain).

Stevens would have been perfectly correct if he had simply said that the screen gives off a very different distribution of wavelengths to the lemon, and that our visual system only detects what is called the dominant wavelength of light, not the actual wavelengths present.

"Our retinas contain three types of cone cells that are receptive to colour, and each one is best suited to detect a certain colour. One is great for blue, the other is great for green and the third is great for red. Notice that there is no individual cell looking for yellow."

The modern scientific explanation of colour vision, as set out on the preceding page, combines the trichromatic (three cone) model with the opponent model of hue perception. Like many other popular explanations of colour, Stevens describes the trichromatic input of the three cone types, but makes no mention of opponency, and in fact offers no explanation of where colours themselves actually come from. This omission, and his choice of words here, create the impression that colours exist in light irself and are 'detected' by the cones.

Apart from the wording that cone cells respond to colours rather than wavelengths, this part is a fair description of the original 19th century version of the trichromatic model of colour vision. We have since discovered that the long-wavelength (L) cone is actually most sensitive to the yellow band of the spectrum, a fact that is even shown in one of the diagrams that Stevens displays. However it remains true that a blue, green and red light will respectively cause the S, M and L cones to each respond more than the other two.

"All a computer monitor or mobile phone screen has to do to make you think you're seeing yellow is send a little bit of red and a little bit of green light at you. As long as the pixels and the little sub pixels on them are small enough that you can't distinguish them individually then your brain will just say well I'm receiving some red and some green, that's what yellow things do, hmm, it must be yellow, even though it actually is not."

Stevens presents here a description of colour vision in which colours exist physically in the spectrum, and the brain identifies colours based on the colours it supposedly detects with individual cone cell types. He assumes that the perception of yellow is an attempt to identify "real" yellow, meaning the wavelengths of the spectrum we see as pure yellow, and therefore considers the brain to be "lied to" when it sees other combinations of wavelengths as yellow.

The modern scientific view of colour vision instead explains how the spectrum of colours is a creation of the eye and brain (see "Summary" below). The brain can not "say" it's receiving some red and some green, becuase no cone cell can "detect" any particular colour band of the spectrum. The yellow colour signal is not a guess that light is from the narrow band of wavelengths we see as pure yellow, but is the response of the visual system to all middle- to long-wavelength light. A balanced response of the L and M cones produces pure yellow because the red/green colour signals cancel out.

Colour Mixing: The Mystery of Magenta (Steve Mould, The Royal Institution, 2013)

"Purple is a weird colour" ... "your brain invents a colour, it makes up a colour, and that colour is magenta"

Mould is perfectly correct in saying that the brain "makes up" magenta; the problem is that his statement leaves the impression that the colours of the spectrum are not 'made up', and so must 'exist' in light itself. The notion that magenta alone is not "real" has never been accepted science, but is encountered elsewhere on the internet (see Michael Moyer's Stop This Absurd War on the Color Pink). The idea is apparently based on the assumption that the spectral colours are real because they "exist" in individual wavelengths of light. Magenta lacks these credentials and so is said to be "made up" by the brain when it looks at mixtures of wavelengths from the two ends of the spectrum. This argument itself seems a bit "weird" because magenta gives every appearance of being simply a mixture of the red and blue or violet colours supposed, on this assumption, to "exist" in those light mixtures. However magenta actually is "made up" by the brain, but so are all of the spectral colours as well. In the modern theory of colour vision, all colours are created by the brain in exactly the same way, as combinations of yellow/blue and red/green colour-opponent signals based indirectly on unequal responses of the cone cells, regardless of whether the stimulus is a single wavelength or a mixture.

Mould's essential point here is that the colour of a mixture of wavelengths from the ends of the spectrum is not the colour of the average of these wavelengths (green), but is an entirely different colour. This could be less problematically expressed by saying that colour is the mental perception of what is called dominant wavelength, which is not a simple the average of the wavelengths present, but has a 360 degree range, such that balanced mixtures of all wavelengths have no dominant wavelength, and appear white. (If dominant wavelength was a simple average, balanced mixtures of wavelengths would also appear green). The reason why this happens rests on the very factor that is missing from Mould's explanation - the opponent processing of cone responses, beginning in the retina.

"... you have these cone cells at the back of your eyes that are sensitive to different parts of the spectrum, so when red light comes into your eyes there's a set of cones that fire and tell your brain you're looking at something red, so we'd call those the red cones. There's another set of cones that are more sensitive to green, so when there's green light going into your eyes they fire and they send a message to your brain, and there's blue cones as well. So you've got red cones, green cones, and blue cones. So what about yellow? What about when you're looking at yellow light, like that? Well in that situation, you don't have a yellow cone. So what do you do? Well, yellow is quite close to red so your red cone fires a bit, and yellow is quite close to green as well so your green cone fires a bit ... So your brain is getting a message from your red cone and your green cone at the same time and it's deciding OK well I must be looking at something in between those two colours, and that's brilliant because your brain is perceiving something about the world that it isn't able to measure directly; it isn't directly sensitive to yellow light."

This part describes the 19th century version of the trichromatic model of colour vision, before it was discovered that the three cones have surprisingly broadly overlapping sensitivities, that the L cone actually responds most to the yellow band of the spectrum, and that the brain does not get "messages" directly from the three cones, but creates colour signals based indirectly on impulses from the retina that convey differences between pairs of cone respones. Like Stevens, Mould makes no mention of opponency.

It can be seen from the excerpt as a whole that the words "isn't directly sensitive to yellow light" are just very carelessly chosen, and that Mould is aware that the so-called "red" and "green" cones do in fact respond to light from the yellow band of the spectrum (though apparently not that the so-called "red" cone actually responds most to the yellow band). These words however gives the highly misleading impression that the cones each respond to a particular colour band of the spectrum, and don't respond at all to other bands. Indeed they even seem to have misled the writer of the caption to this video on its YouTube page, who says (completely incorrectly): "The cone cells within our eyes ... are only sensitive to Red, Green and Blue light. So how are we able to see so many colours when we can only directly detect three...". This particular misconception now seems quite common, and may stem directly from this source.

One could avoid introducing nonsense about cones being sensitive to red and green but not to yellow light simply by saying that the relative response of the L and M cones changes progressively through the long-wavelength part of the spectrum, and that this progressive change in the difference between the L and M cone reponses is experienced as the colours red, orange, yellow, yellow-green, and green.

"It does mean that you can be tricked. ... there's no yellow light here, but you'll see yellow anyway"

Like Stevens, Mould means by this that wavelengths from the yellow band of the spectrum are entirely absent from his mixture of red and green lights, which is probably not strictly correct, but the actual issue here is the tacit assumption that seeing yellow is an attempt to identify a particular narrow band of wavelengths of the spectrum, and hence that we are "tricked" when we see a mixture of red and green wavelengths as yellow. Once again, yellow is a perception, not a wavelength, and both yellows are equally real perceptions. It would be more accurate to say that we are all tricked into assuming that yellow is a physical property residing in light, when really it is a mental perception caused by many physically different lights that happen to have the same effect on our cone cells.


As proposed by Young and Helmholtz in the 19th century, the trichromatic theory held that three receptors sensitive to the parts of the spectrum seen as red, green and blue or violet generated fundamental sensations, and explained the sequence of spectral colours as successive combinations of these three sensations in changing proportions. Yellow was considered to be one such "compound" sensation composed of red and green sensations. In the modern zone theory, an updated form of the trichromatic receptor model is coupled with the opponent model of colour perception. The L, M, and S cones respond most strongly to the parts of the spectrum seen as yellow, green and blue-violet respectively, and the brain creates yellow/blue and red/green opponent colour signals based indirectly on signals representing differences in the cone responses. The sequence of colours of the spectrum is explained as the product of successive combinations of these red, yellow, green and blue opponent hue signals.

The two YouTube videos incorporate the receptor sensitivities envisioned in the 19th century version of the trichromatic model, but make no attempt to explain the sequence of colours of the spectrum as combinations of either three "fundamental sensations" or four opponent colours. In fact they give no indication at all that these colours originate in the brain. Instead they couple the early trichromatic receptor sensitivities with explanations that speak of the spectral colours as if they are physically real and reside in light itself. The omission of any explanation of the sequence of colours in the spectrum adds to the impression that this sequence simply exists there.

This combination of premises leads both Stevens and Mould to offer an inverted explanation of the perception of yellow that bears little resemblance to present or past science. Real yellow is deemed to reside in the spectrum in the wavelengths we see as pure yellow. Although there is supposedly no cone that is "looking for" (Stevens)/"directly sensitive to" (Mould) these pure yellow wavelengths, the brain somehow knows of the existence of this "real" yellow in the spectrum, and its location between red and green. The perception of yellow is treated as a process of identification of this invisible, externally existing yellow by a defective criterion that can result in misidentification. This criterion is the balanced response of the so-called red- and green-detecting cones.

In the modern scientific explanation the L cone (their "red" cone) is known to be most sensitive to the yellow band of the spectrum, but much more to the point, the L cone is not a yellow 'detector' or a red 'detector'. Instead, the L and M cones detect essentially all wavelengths of visible light, and high L and M cone responses both contribute to creating a yellow colour signal throughout the middle and long wavelength parts of the spectrum. Through most of this range this yellow signal is combined with either a red signal (where the L cone responds most) or a green signal (where the M cone responds most). Pure yellow is seen when the L and M responses are balanced and the red/green signal cancels out. This occurs in the spectrum in a narrow range of wavelengths near 570 to 580 nanometres, but also for exactly the same reason in any mixture of wavelengths that has the same relative effect on the L and M cones.

It's likely that Stevens and Mould were not especially concerned with colour itself, and mainly intended to make the valid point that physically the brain effectively determines the dominant wavelength of light by means of the relative response of the three cone types. It's just unfortunate that in doing so they create or perpetuate misconceptions about the fundamental nature of colour, the details of the colour vision process and (in the case of Stevens) the physical basis of object colours. The combination of outdated cone physiology and colour "realism" that these videos reinforce constitutes perhaps the most widely-held view of colour vision among non-scientists today. Although the details of how the colour-opponent signals are generated within the brain are still mysterious, the opponent model itself has been widely accepted in science for many decades. In selecting the topic "What is color?", the 2014 Flame Challenge has certainly highlighted an area where scientists have largely failed to convey a modern understanding of the topic to a wide audience, and apparently even to some science communicators.

March 9, 2014.

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Next: Part 4: Additive Colour Mixing