Colour spaces

PART 1. THE DIMENSIONS INTRODUCED

1.1 Colours in Space

Hue, Value and Chroma

All painters, whether working in traditional or digital media, are in a real sense navigators in space. Whether they are aware of it or not, each touch of colour they apply can be considered, using various systems, as a point within a space defined by three dimensions.

Figure 1.1.1. Left: Portrait of Vincent Van Gogh by Henri Toulouse Lautrec. Pastel, 1887. Right: RGB colours from this image, plotted in YCbCr colour space using the program ColorSpace by Philippe Colantoni. (www.couleur.org).

The three-dimensional system most familiar to painters is the classification of colours of objects according to dimensions of hue, value and chroma. Hue refers to the circular scale of "pure" or "saturated" colours formed by the colours seen in the spectrum (red, orange, yellow, green, cyan, blue and violet), together with the non-spectral colours like magenta, seen when the two ends of the spectrum are mixed. Value refers to the scale from black to white; tone, greyscale value, and lightness are synonyms or very closely related. Chroma or colour strength refers to the amount of visual difference from a grey of the same value. In short then, the system may be said to classify any object colour according to the closest full or "saturated" colour, the closest grey, and the visual difference from that grey (Fig. 1.1.2). Because the maximum chroma attainable with any set of paints varies with value, a hue page, which shows all variations of value and chroma for a given hue, has an irregular right margin (Fig. 1.1.2), and because this margin varies for each hue (Fig. 1.1.3), a hue-value-chroma space has an irregular, tree-like shape.

Figure 1.1.2. Explanation of the dimensions of hue, value and chroma in the Munsell system. Grey scale and 10YR hue page from the Munsell Book of Color (glossy edition).

This three-dimensional classification and its key concept of chroma were devised by the American artist and art teacher, Albert Munsell (1858-1918), and published in a small book entitled A Color Notation (Munsell, 1905). Munsell published quantitative scales of hue, value and chroma in an Atlas of physical colour chips (Munsell, 1915), which after his death was elaborated by the Munsell Color Company (directed by his son Alex) as The Munsell Book of Color (1929). This was further refined and developed by the Optical Society of America, culminating in the “renotation” published in 1943, which related the Munsell System to the standard system of colorimetry developed by the Commission Internationale de L'Eclairage (CIE). This 1943 "renotation" forms the basis of all subsequent editions. The Munsell Book of Color has 40 hue pages (Fig. 1.1.3), and is available in a choice of editions having either matte or glossy colour chips. An alternative hue-value-chroma system based on CIE L*H*c space is also available as a physical atlas of colour chips, the RAL Design Atlas. Hue, lightness (value) and chroma are included by the CIE as three of the six defined attributes of perceived colour, of which the Munsell and CIE L*H*c systems provide two alternative sets of quantitative measures.

Figure 1.1.3. The forty hue pages of a modern edition of the Munsell Book of Color (glossy edition). Click on each hue page to enlarge, and scroll down for more pages. In the matte edition the range of colours tends to be a little greater among the light colours and a little less among the dark colours.

The dimensions of value and chroma apply specifically to colours perceived as belonging to objects, as opposed to lights. Colours perceived as belonging to lights (Fig. 1.1.4B) can be described in terms of three dimensions of hue, brightness (perceived amount of light) and either saturation (colour purity, i.e. perceived freedom from admixed white light) or colourfulness (colour strength, a function of both brightness and saturation).

Figure 1.1.4. Attributes for colours perceived as belonging to (A) objects and (B) light. Saturation refers to purity of colour of light, and can vary throughout its range (white to monochromatic) at any level of brightness; it is represented in B by the angle from the neutral axis. Colourfulness refers to strength of colour of light, and can be thought of as saturation times brightness; it is represented in B by the distance from the neutral axis. Chroma (strength of colour of objects) depends on the colourfulness (saturation and brightness) of the light given off by an object for a given level of illumination. Chroma is necessarily zero at maximum and minimum value (white and black respectively), and reaches its maximum range at intermediate value levels.

Value and chroma apply to colours of objects seen in nature or depicted in an image, as well as to colours of an image itself. This is true whether the image surface reflects light (e.g. a photograph, painting, or projector screen), transmits light (e.g. a stained glass window) or emits light (e.g. a computer monitor or TV screen). However, areas of the visual field occupied by objects can also be seen as light, and thus the dimensions of brightness, saturation, and colourfulness apply not only to primary light sources but also to the light remitted by non-luminous objects to our eyes.

Painting Colours in Space

Any painter familiar with the colour wheel and tonal scale of traditional "colour theory" would surely be aware to some extent of the spatial aspect of their activities, but many do not seem to derive much advantage from this awareness. Such painters may for example vaguely observe that a given colour needs to be "warmer" or "cooler", without troubling to apply the more precise concepts of hue and chroma. Many think of colour mixing as applying colour "recipes", obtained either secondhand from art instruction books, or from their own elaborate mixing charts showing how they mixed a particular colour previously. Typically, painters of this sort have little knowledge of the physical principles involved in creating effects of light and shade, and often rely for such effects on crude and inaccurate formulae, such as to "get the shadow colour by adding the complementary colour", and so on. A hallmark of this uninformed approach to colour mixing is the traditional admonition "Don't use black!". The real problem is not black paint, but the painter's inability to visualize any unintended effect of adding black paint as an easily corrected shift within colour space.

In contrast, other painters find it invaluable to think consciously of their colouring activities as maneuvering through a three-dimensional colour space. Most think of this space in terms of relative hue, value and chroma, but there others who train themselves to think in terms of absolute scales of these dimensions such as those of the Munsell Book of Color (q.v. Graydon Parrish and Steve Linberg's Classical Lab). The glossy version of the Munsell "big book" is favoured over the matte version among oil painters because paint mixtures can be tested on the individual removable colour chips and then wiped off safely. Painters who stop short of going the full Munsell often find it very helpful to at least think of value in terms of an absolute scale of some kind.
A three-dimensional conception of colour assists painters by providing a framework for observing colour relationships, for selecting and mixing colours, and for creating colour relationships from the imagination.
1. As a framework for observing colour relationships.
Most painters of this latter kind do not try to copy each colour in their subject in isolation (the strategy of every beginner). Instead, they use the concept of colour space as a frame of reference for grasping the relationship of each colour to the totality of colours present. Tonal realist painters, for example, typically observe colour relationships in the light from their subject, and then, by a process of either conscious or unconscious translation, identify each individual colour in terms of the hue, value and chroma of the paint colour they will need to use in order that the whole ensemble replicates the visual appearance of the subject as closely as possible. In practice, this usually involves first selecting the most important ten or so colours in the subject, and finding the place of these in relation to each other (Fig. 1.1.5). This begins the process of building what I call a scaffolding for progressively finding the place of all remaining colours, most of which can usually be considered as variations on, or intermediates between, these scaffolding colours.

Figure 1.1.5. : Left: Lyndall by David Briggs, 2005, oil on canvas. Right: plan view (above)
and side view (below) of ten selected colours from the image plotted in YCbCr space using
the programme ColorSpace by Philippe Colantoni. (Note that, as in most illustrations on this
site, the CbCr plane is shown in reverse to its standard orientation, to place the spectral
sequence of colours in clockwise order, following Newton, Munsell, and the hue circle in HSB
colour space, among others).

2. As a framework for selecting and mixing colour.
Artists who think in terms of colour space do not need to remember recipes for mixing colours: they understand that most colours can be mixed from any number of combinations of paints, as long as the target colour is within the three-dimensional gamut of those paints. They literally visualize colour mixing as moving colour from place to place through colour space. They decide on the changes in hue, chroma and value required, and predict in advance what effect various additions are are likely to have. These crafty painters often, for example, premix a pool of colour on the other side of a target colour, and add this in stages to draw the colour methodically towards its target (Fig. 1.1.6). This approach to colour mixing was developed to an elaborate degree by the influential mid-20th century American teacher Frank Reilly, whose approach has been described in books by his ex-students including Apollo Dorian, Frank Covino, Jack Faragasso, and Angelo John Grado.

Figure 1.1.6. A very simple example of the use of the concept of colour space in colour mixing. The target colour (B) is observed to have higher lightness and lower chroma than the starting colour (A). Adding white to A is expected to make both of these changes, but when tried is found to produce mixtures that are still too high in chroma when they reach the lightness of B. Pre-mixing a pool of light grey at around the lightness of C, and adding this in stages to A, should bring the mixture closer to the target.

3. As a framework for creating colour relationships from the imagination.
The dimensions of colour form an essential conceptual framework for any kind of activity that involves creating colour relationships from the imagination. In the nineteenth and early twentieth centuries, much thought on colour spaces (including Munsell's own writings) was directed towards discovering rules of "colour harmony", and there are still many echoes of this kind of investigation today. On this site however I am much more concerned with the application of colour space to the creation of convincing effects of light. The concept of colour space provides a quantitative framework for applying the simple physical laws that govern the behaviour of light and colour, some of which were understood in a qualitative way as far back as Leonardo. If the artist gets these relationships right in a painting, the payoff is not merely technical correctness but can be a vivid glow of light and feeling of atmosphere. And, just as with, for example, perspective and anatomy, having the understanding that allows you to do something from the imagination makes working from nature far more efficient.

Figure 1.1.7. Imaginary sphere under three imaginary light sources, painted as three layers in screen mode (one for each light source). David Briggs, 2007, Photoshop CS2.

In the remainder of this introductory section we will examine each of the major dimensions or attributes of colours in turn, but in order to really understand our subject we must first take up the thorny question of what these "colours" actually are!
 
 
 
Modified January 14, 2017. Original text here.


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