If you have white and black of the same type of paint, mix your own. I do this all the time as I hate to stock lots of infrequently used colors. A couple of variants on each of the main colors and a few of the very hard to mix (e.g. neons) and I can get pretty much any color I want.
A great book on color mixing is "Blue and Yellow Don't Make Green", by Wilcox. It's oriented around artists colors, not model paints, but the principles are universal.