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.
