Mine has continued to stay bright yellow, blue polkadots and a little brown mixed in here or there. I have had him almost two years. What are you feeding him? Mine loves a variety of meaty foods(brine, mysis, squid, etc...) but they also need a lot of veggies(seaweed, nori, spinach) are all foods that mine likes to eat. Maybe dose with some garlic and/or vitamins. If your water parameters are good then I would have to guess stress or nutrition would be causing your color change.