Black and white's always safe. Using one color, I like a low contrast effect (ex. a warm green set against a lighter tint for text). With two, working with natural values, yellows never dominating the greens or blues etc. No combination of red, blue, yellow, or green and black/white. I play a long time with whitish yellows, blues, and greens, all the different tints of these three colors, while with red I'm always drawn toward a weathered brick color, a real low saturation red, sometimes almost gray (I never use any 'true' gray values, but do like the borderline grays of blue and red for background).