John- If you're willing to experiment, in addition to what you've mentioned, try oils, food coloring, dyes, and pencils (if you want textured color). Whatever you use, you'll need an even density if you want an even tone on the positive. Q-Tips, and cotton balls, will even your application.
You could always print a bunch of step wedges, and color each of them with one color. Then, you'd see how each color lays (and prints) on a particular tone.