The Testors will sure work, but what I have used is artist pastels. It comes in sticks as chalk and you shave off dust with a knife blade into the epoxy. Buy it at art supply stores.
White is suprisingly difficult to match perfectly. There are a lot of different shades of white! If you are lucky and the bottle of Testors matches well for you that would be the cheapest option. But for doing a wide range of colors and matching shades exactly I found the pastel chalks give better variety.