Nice! I made something similar to teach myself Qt. It took an image and turned it into an offset grid of circles. You could adjust the area of the image assigned to each cap to see how many caps you need to get something recognizable. Then it would spit out a listing of how many caps you need for each color.

Just a heads up: the number of caps needed to make a recognizable david tennant in front of the tardis is very high. Table size becomes an issue. :)