Dumb question, but I know nothing about fabrics. I want to dye my computer sleeve (organic cotton canvas) a different color but I don't want to do so if its going to look terrible. Its currently dyed a brownish color and I want to dye it dark black. Can I do this since its already been dyed a different color? Will the dyes mix and give me a brownish black or if I leave it in the water/dye solution long enough will become only black?