From A List Apart –Contrast is King:
Being colorblind doesn’t mean not seeing color. It means seeing it differently. If colorblindness challenges the colorblind, it also challenges designers. Some of us think designing sites that are colorblind-friendly means sticking with black and white, or close to it. But the opposite is true. Using contrast effectively not only differentiates our site’s design from others, it’s the essential ingredient that can make our content accessible to every viewer, including the colorblind. By understanding contrast, we can create websites that unabashedly revel in color.