yellow belly sliders

Question
I hope you can help me. I have two very young yellow belly sliders, and when I first got them their bellies were very yellow, now not so much. I have a heat lamp, a water heater, and a UV ray lamp. I keep the lamps on for about nine/ten hours, then I keep only the water heater on over night. I even put reptisafe in the tank every time I clean it. I feed them mostly  aquatic turtle food from the pet store but I feed them shrimp sometimes. I don't know what I am doing wrong that their bellies would change color. The color is a very mild yellow almost white. I know they are yellow belly sliders, so I don't know what I am doing wrong. If you could please give me some advice, I mean they seem healthy, they swim, they dry themselves off they eat just fine, so why the change in color?

Answer
Sometimes as animals grow especially reptiles they can and do sometimes lose the coloration over time. If you could tell me what the heat is at for the heat lamp and the water that may be part of it but I doubt that very much. The authority on turtles would be Marc Oullette of http://www.littleresq.net/ he is a great friend of mine and is always happy to help.