This issue comes up a lot, and I always tell people not to be concerned unless it goes on for quite a while and/or there are other signs of stress or illness. Remember that bright colors are a way that chams communicate stress, dominance, and breeding availability. Dull colors can be due to a range of reasons and it's impossible to know which is the cause without knowing a lot about the husbandry of your setup, the diet, lighting, supplementation, temperatures, age, stress factors in their setup area, underlying health issues that may not even be showing other symptoms, and many more possible factors. It could even be that you just have a happy, relaxed cham. Like I say, it's really hard to say.
It's good that you care. And by all means, keep observing for issues or problems. Just don't worry to the point that you try treating without really knowing what the issue is...or isn't. In most cases that can cause more problems than it fixes.
Just my mostly unsolicited $.02!