Water based craft paints. Walmart sells a lot of colors in bottles that are around 8 ounces. Inexpensive at under $2 each.

I painted a Simple 400 wing and it only took an ounce or so, and covers quite well. It will brush on and look fairly good, but you're adding more weight with a brush; if you have an airbrush, this stuff could be thinned and sprayed on for very little weight gain.
Dennis-