just feed your yellow tang well and keep it happy, it will retain its bright colors. Lots of dried seaweed (called nori, you can get it at your local oriental food market) and a homemade blender mush will do wonders. (do a search on the net for marine mash or blender mush and get a detailed recipie, but its basically throwing some chopped peeled raw shrimp, mussels, clams, vitamins like zoe or selcon in the blender and grind it up, freeze it flat and break off a chunk when you want to feed it.)