Simultaneous Contrast

July 2019:
Recently I was reading an article on colour written by Charles Blanc in 1867.   Blanc, a friend of Manet’s, was for several terms, director of the Ecole des Beaux Arts in Paris.  In his article, Blanc points out that complementary colours placed side by side will enhance the appearance of each other.  This, of course, had been known for a long time, but it was M. Chevreul,  (A French chemist whose colour theories influenced French painters) who put a name to the effect: Simultaneous Contrast

Artists have long used simultaneous contrast to enhance their colours. For example, a green can be made to appear more vibrant if placed nearby a bright red, but it is very important to recognize that the effect works both ways, hence the term ‘simultaneous’. Blanc also noted that the same colours that heighten each other through juxtaposition, will destroy each other when mixed together.  Red and Green enhance each other when side by side, but turn into a dull grey when mixed…hence another reason for the double, or simultaneous contrast. The effect can also be observed with similar colours. For example, a bright blue placed beside a dull blue will appear more vivid and the dull one more dull.

Blanc also told about Eugene Delacroix, the great colourist, who while painting yellow drapery was unable to achieve the brilliant yellow he wanted.  Putting down his brush, he ordered a carriage to go to the Louvre to see how Rubens and Veronese achieved their brilliant colours. This was 1830 and there were many canary yellow cabs in Paris at the time.  As he was about to step into the yellow cab, he stopped; to his great surprise, the yellow of the carriage had produced violet in the shadows.  He cancelled the cab and returned to his studio to apply what he had just witnessed. 

A similar effect can be seen if you have a red curtain with a tiny hole in it.  A white paper held near the hole will display a small green dot where the sun, passing through the hole, hits it. This occurs because the paper as a whole is illuminated by red light from the curtain, making the bright dot seem green via simultaneous contrast.

In all of these effects the characteristics of the light do not change; there was no violet in the shadow that Delacroix witnessed.  There is no physical means by which placing red beside green will affect the light reflected from the green pigment. What actually happens is that the red receptor cones in the eye excited by the red light actually cause a decrease in the sensitivity of the surrounding red cones.  For reasons having to do with the complex way the eye combines the three colour receptors (red, green and blue). The effect, it seems to me anyway, is most noticeable for the red- green complements and much less so for the blue-orange and yellow-violet complements.

This simultaneous contrast effect may also be the source of the Impressionist’s belief (which erroneously persists to this day) that shadows are the complement of the yellow sunlight (ie purple). There is no physical means by which this is possible. However, it is possible for the eye to interpret the shadow as purple if it is cast by green trees, which was more often than not the case for the Impressionists. The simultaneous contrast effect may well make the shadows appear somewhat purple by decreasing the eye’s sensitivity to the green light in the shadow area. This effect could also be enhanced by the fact that the light illuminating the shadow area will be the blue light scattered by the atmosphere (only if the shadow area is open to the sky but blocked from the sun). However, the effect is subtle and can easily be overpowered by light quality and ground colour. If you are out on a ski trail sometime on a sunny winter day, look closely at the colour of the shadows. Because of the white snow, shadow colour is easy to determine. Shadowed areas that are blocked from the direct sunlight but are still open to the sky will be blue because of the blue scattered sunlight. Shadow areas that are blocked both from the sun and open sky will appear grey and, of course darker, being simply some residual light reflected from the surrounding trees etc.