PART 1. INTRODUCTION: THE DIMENSIONS OF COLOUR

COLOURS IN SPACE

All painters, whether working in digital or traditional mediums, are in a real sense navigators in space. Whether they are aware of it or not, each touch of colour they apply can be represented, using various systems, as a point within a space defined by three dimensions. The conceptual colour space most familiar to painters, which is applicable to colours perceived as belonging to surfaces, has the dimensions of hue,  chroma (strength of colour) and lightness (the latter usually known to artists as tone, greyscale value, or simply value). Colours perceived as independent light, on the other hand, should be described in terms of the fundamental dimensions of hue, saturation (purity of colour) and brightness. A sixth parameter, "colorfulness", used in current literature on colour appearance models (Fairchild, 2004, 2005), is a function of both the saturation and the brightness of a light stimulus.
  

Figure 1.1. Left: Portrait of Vincent Van Gogh by  Henri Toulouse Lautrec. Pastel, 1887. Right: Colours from the Lautrec pastel plotted in a hue-chroma-lightness colour space (YCbCr), using the program ColorSpace by Philippe Colantoni (www.couleur.org).

All painters familiar with the concepts of the colour wheel and the tonal scale must presumably have some awareness of this spatial aspect of their activities, but many do not seem to take much advantage of this awareness. Beginners often seem to mix colours entirely by trial and error, with little thought of what effect a colour is likely to have before they add it. Many painters observe colours as being in a vague sense "warmer" or "cooler" than others, but do not trouble to apply the more precise concepts of hue and chroma. A great many think of colour mixing as applying pigment "recipes", obtained either secondhand from art instruction books on, say, how to paint flesh colours, or by relying on their own mixing charts to remember how they mixed a particular colour previously. Typically, artists 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 rules of thumb, such as "get the shadow colour by adding the complimentary" or "keep your color most intense on the edges of the lighted areas".

Other artists however find it invaluable to think consciously of their colouring activities as maneuvering through a  three-dimensional space. They use this spatial conception in three main ways, as a framework for (1) observing colour relationships, (2) selecting and mixing colour, and (3) creating colour relationships from the imagination:

1. As a framework for observing colour relationships.

Most artists of this group 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 relationships of hue, brightness and "colorfulness" 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, lightness and chroma of the paint colour they will need to use in order that the whole ensemble replicates the visual appearance of the subject. In practice, this usually involves first selecting the most important (say) six to ten colours in the subject, and finding the place of these in relation to each other (Figure 1.2). This begins the process of building what I think of as a scaffolding for progressively finding the place of all remaining colours.

Most of these painters utilize some system of premixed pools of paint. Many make their colour decisions in relation to a fixed system of colours, usually either a published colour classification like the Munsell system, or some other methodical procedure for laying out a standard set of predetermined colour mixtures. Others think instead in terms of hue, chroma and lightness diffrences relative to their "scaffolding" colours. Many painters who stop short of going the full Munsell nevertheless find it invaluable to at least think of lightness in relation to a scale of (usually about nine) absolute greyscale values.

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 components, as long as the target colour is within the three-dimensional gamut of those components. They literally visualize colour mixing as shifting colour from place to place through coour space. They decide on the changes in hue, chroma and lightness required, and predict in advance what effect various additions are are likely to have. These crafty painters may, 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 (Figure 1.3).

Figure 1.3. 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 was directed towards discovering rules of colour harmony, and there are still many echoes of this kind of investigation today. Here I am much more concerned with the relevance of colour space to the creation of convincing effects of light from the imagination. The concept of colour space provides an essential quantitative framework for applying the simple physical laws that govern the behaviour of light and colour. If the artist gets these right in a painting, the payoff can be a vivid glow of light. And, 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.

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Figure 1.4. Imaginary sphere under three imaginary light sources. Painted as three layers in screen mode (one for each light source) in Photoshop CS2.

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