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.)
We've Become Accustomed to Expressionism
Remember that a lot of what we're used to was new at the time of the Expressionists. When you look at Matisse's Girl with Green Eyes painting, for example, it's hard to believe his contemporaries were outraged by it and regarded it as grotesque. Matisse biographer Hilary Spurling says: "The confident gaze and frank body language of these young women, painted almost a century ago, speak directly to us today, although contemporaries could see little in these portraits but meaningless jumbles of color outlined in ugly black brushstrokes."3
In his book Bright Earth: The Invention of Color, Philip Ball writes: "If Henri Matisse made colour the substance of pleasure and well-being, and Gauguin revealed it as a mysterious, metaphysical medium, van Gogh showed color as terror and despair. Munch's remark apropos of The Scream (1893) that 'I ... painted the clouds like real blood. The colours were screaming' echoes van Gogh's sanguine comment on The Night Cafe -- 'a place where one can ruin one's self, go mad, or commit a crime'."4
How to Paint Like an Expressionist
All that said, how would I approach trying to paint like an Expressionist? I'd start by letting the subject matter of the painting determine the colors you select. Go with your instinct, not your intellect. Initially limit the number of colors you use to five -- a light, medium, dark, and two tones in between. Then paint with them according to tone not hue. If you want to use more colors, I'd start by adding complementaries. Use the color straight from the tube, unmixed. Don't second guess yourself until you've done quite a bit of painting, then step back and look at the result. For more, see How to Paint in an Expressive or Painterly Style.
Take a look at the paintings from the Van Gogh and Expressionism exhibition for inspiration or use one of the paintings as a starting point for one of your own. Copy a painting and then paint a second version without looking at the first, entirely from memory, letting it go where it wants.
1. Matisse the Master by Hilary Spurling, page 26, Penguin Books 2005.
2. Van Gogh's letter to his brother Theo from Arles, dated 11 August 1888
3. Matisse and His Models by Hilary Spurling, published in Smithsonian Magazine, October 2005
4. Bright Earth by Philip Ball, page 219.