Saturation and relative brightness turn out to be vitally important parameters when it comes to understanding colour and light, and when creating effects of light from the imagination. In the final section of this work I will summarize what seem to be the main considerations under eight major principles. Many of these principles have been set out somewhere or other before, most notably I believe in a series of papers by Arthur Pope (1922, 1931), but they deserve to be a lot better known than they are.


Uniformly coloured surfaces passing into different levels of illumination are
represented by colours that move along a line of uniform saturation.

If a surface of uniform colour turns progressively away from a single light source, the diffuse reflection from its surface steadily decreases in brightness. We would of course expect to represent this in a painting with a series of colours diminishing in lightness, but what would be the chroma of these colours? The answer lies in the fact that because the colour of the illuminant and the colour of the surface are both constant, the proportion of the different wavelengths in the reflected light will not change. Consequently the hue and saturation of the reflected light remain constant, while the brightness diminishes. The series of colours we use to represent such a surface, here called a shading series, should therefore lie along a line of uniform saturation; such lines radiate from the black point of the colour solid (Figure10.1). Along such a line, chroma decreases steadily as lightness decreases, at the precise rate necessary to keep the saturation of light from the surface constant.

In digital work we can easily create such a series by keeping the hue angle and percentage saturation constant (in effect keeping the ratio of R/G/B constant), while the relative brightness (B) decreases. Uniform saturation series are easily created in Photoshop using the colour picker, which allows S and B to be directly manipulated. In Photoshop shading series can also be created by various shortcut methods such as

  1. varying the opacity of a layer in normal mode over a layer of black, or
  2. varying the opacity of a layer of black in multiply mode over a layer
  3. by placing a coloured layer in multiply mode over a greyscale image.
    The first two methods work in RGB mode but not Lab mode.

In Figure 10.1 Principle 1 is used to draw a red sphere from the imagination. The highlight retains the (white) colour of the light source. Around the fringes of the highlight, additive mixing of this white light with the red diffuse reflection creates intermediate colours, while the darker versions of the red are restricted to the crevice shadow.

Also note that, contrary to a widespread myth among painters (e.g. Loomis, 1947), the richest colour is not on the edges of the lighted area. In the case of Loomis this belief is connected with his belief that the highlight coincides with the centre of the full light (link).

Figure 10.1. Colour relationships for a red ball on a white table.  Specular reflection on tabletop and sphere both move along lines of uniform saturation between light and dark (Principle 1). Dotted lines shows table and ball maintaining the same ratio of relative brightness in light and shadow (Principle 2). Sphere painted in Photoshop CS2.

According to this principle, B = 100 is the brightest version of any colour that can be depicted- no colour in RGB space has a greater brightness and the same balance of wavelengths. We will in fact see that even painting a surface of full chroma, we will generally not use colours as bright as B = 100 if we wish to leave room to represent the highlight. The only way to go lighter than this is to try to give the effect of a very bright light beyond the range of adaptation of the eye, and go to paler, lighter colours, like those seen in an overexposed photograph.

In traditional paint mediums, the colour relationships discussed here need to be established by eye. With practice it is not difficult to create a shading series in paint. Such series are not created by just adding black paint. Black pigment tends to reduce the chroma of mixtures more rapidly than the lightness, creating colours that are too neutral (grey) for their lightness level - hence the hobby painter's insistence on not using black, because it "muddies" the colours. My advice is that anyone who doesn't understand colour should not use black, but now that you've got this far you're certainly ready to join the black-using party, who incidentally form the majority of great painters in every period of art history. You understand by now that painting is about creating colour relationships, not about following colour recipes. In this case, the solution is to add just enough of pure colour to get the mixture back onto a uniform saturation series (Figure 10.2). Some fine hue adjustment may also be needed to counter any hue-shifting effects of the black, but the same is true whatever pigment is used for darkening.

Figure 10.2. Adjustment of saturation with pure red pigment after adding black pigment. Adding black to a mixture of Permanent Alizarin Crimson and White draws the mixture away from the shading series into colours that are too low in saturation. The solution is to add just enough extra Permanent Alizarin Crimson to get the mixture back onto the line of uniform saturation. Some fine tuning of the hue is also generally needed.

Incidentally, adding the complementary colour, the hobby painter's usual alternative to adding black for darkening, works tolerably well for some colours for a small amount of darkening and then starts drawing the mixture in the wrong direction; for other colours it draws the mixture in the wrong direction right from the start (Figure 10.3).

Figure 10.3 Mixing of Cadmium Red and Cobalt Green). Cadmium Red and Cobalt Green are near complementary in pigmentary mixing (B). Cadmium Green initially draws Cadmium Red along a line of uniform saturation, then begins drawing the mixture inward to less saturated colours; Cadmium Red however draws Cobalt Green away from the required shading series (black arrow) right from the start.

<< 1 2 3 4 5 6 7 8 9 >>