Though I'm sure some art-history buffs are likely to say otherwise, from everything I've read about Expressionism it seems the individual artists now labeled as Expressionists largely made it up as they went along, following their instincts as to what color to use, when and where. The 'breakthrough' was that color didn't have to be realistic. While reference is made to colors having symbolic value, again it seems to me that this symbolism was largely determined by individual artists, and not governed by a rigid set of pre-existing rules.
Matisse believed "the invention of photography had released painting from the need to copy nature", leaving him free to "present emotion as directly as possible and by the simplest means".1
Van Gogh tried to explain to his brother, Theo: "Instead of trying to reproduce exactly what I have before my eyes, I use color more arbitrarily, in order to express myself forcibly. ... I should like to paint the portrait of an artist friend, a man who dreams great dreams, who works as the nightingale sings, because it is his nature. He'll be a blond man. I want to put my appreciation, the love I have for him into the picture. So I paint him as he is, as faithfully as I can, to begin with. But the picture is not yet finished. To finish it I am now going to be the arbitrary colorist. I exaggerate the fairness of the hair, I even get to orange tones, chromes and pale citron-yellow." 2
Kandinsky is widely quoted as saying: "The artist must train not only his eye but also his soul, so that it can weigh colors on its own scale and thus become a determinant in artistic creation". Kandinsky was a synaesthesiac, which would have given him an insight into colors that most people don't. (With synaesthesia you don't just see color, but experience it with your other senses too, such as experiencing colors as sounds or seeing sounds as color.)
