Optical Colour Illusions
Simultaneous colour contrast
Simultaneous colour spreading and isolation
Successive colour contrast
Successive colour spreading
Optical mixing by static mosaic
Simultaneous colour contrast
Another aspect of colour theory is concerned with the illusory or psychological effects of colour. When comparing a number of colour samples edge to edge, for~example, it appears that a colour seen in one comparison may look slightly different in hue, tonal value or colourfulness than it does in another. Leonardo da Vinci was aware of this illusion, and recommended (about 1500): 'If you intend to represent great darkness, it must be done by contrasting it with great lightness; on the contrary, if you want to represent great brightness, you must oppose it with darkness; so a pale yellow will cause a red to appear more beautiful than if the red is beside a purple.'
It was not until the early nineteenth century that Michel-Eugene Chevreul set himself the task of attempting to discover underlying principles from which the results of such illusions might be predicted. Prior knowledge of these effects is especially important to the weaver, for whom his research was originally intended, since mistakes made during tapestry-making cannot be entirely corrected later.
Figure 4. Simultaneous colour contrast
Chevreul adopted the term simultaneous colour contrast to refer to a category of optical illusions in which he included 'all the phenomena of mod)fication which differently coloured objects appear to undergo in their physical composition and in the height of their tone ... when seen simultaneously.' By this he was referring to combinations of colours placed edge to edge to be seen together and all at once. Some of the basic principles he established can be summed up as follows:
1. Where two adjacent areas of colour greatly differ in spectral colour difference or hue, the two colours will usually enliven each other's appearance (Figure 4A).
2. Where two adjacent areas of colour are similar in hue, their difference of hue may appear exaggerated, while the colourfulness of each will tend to be lessened (Figure 4B).
3. Where two adjacent areas of colour differ in tonal value, their difference in tonal value will tend to appear exaggerated (Figure 4C).
4. Where two adjacent areas of colour differ in their degree of colourfulness, the more colourful of the two will tend to appear enhanced, while the other will tend to appear duller (Figure 4D).
Chevreul's observations were revised and augmented by Rood, who confirmed in 1879, 'We can actually change a colour to a considerable extent without at all meddling with it directly, it being for this purpose only necessary to alter the colour which lies adjacent to it.'
