They're the same color dyes that color prints are made with. You can spot color prints with them or colorize b&w white prints. The colors are worked into the emulsion layer, combining with it, not painted on top. You mix the colors the same way you would with a color head when making color prints (CYM). The color is built up gradually, similar to doing a water color painting - large areas are colored with a wash. It's not reversible but you can reduce, or delete, somewhat with ammonia. You can also neutralize a color by adding its complimentary color, I think, and make Grey. It's delicate work, I tried it with little success, but I gave up too easy. I've seen prints colored that way by those who mastered the technique and they looked magnificent. The company that made SPOTONE (Retouch Methods Co. Inc) also sold a set - the one I have.