I add about a tablespoon of cocoa powder per cup of frosting to make black or brown tint. Thin down to correct consistency, and add coloring to adjust tint.
You may add maraschino cherry juice to thin and flavor the red and pink frosting, adding in gel color to get the desired final tint.
We usually make one piping bag with white, and then make cups with paintable frosting in these colors:
white, red, green, brown (chocolate), purple, pink, yellow, and small amounts of black and orange or flesh tones.
