What are your basking temps?
ALso, if its winter and cold where you are, they will often get dull looking.
My adult male veiled has more of a tan bar instead of yellow because its cold, once the summer comes in, he goes back to being a brilliant yellow.
ALso, if your basking temps are too low, that can cause him to be darker as this helps them absorb heat.
how are you measuring the temps?
Also, if you are using a red bulb, just get rid of it. its not needed, and just not worth it.
we all use regular house bulbs for basking, and then use various brands of UVB bulbs.