Well putting them together could be the main reason for the change. Males will darken the beards in the presence of a female, but any stress can have the same effect on the whole dragon not just the beard. Most people keep the male and female separate except for when they want, have dragons in great health and proper age, and are ready for the huge responsibility of having gravid females, and babies. No doubt if your keeping a male and female together eventually you will deal with eggs, and aggression when breeding time comes. Males can be rather domineering to the females. I would separate them if you don't have plans to breed. It seems they do better, and are more likely to keep the light happy colors when kept stress free and alone.
Keep the cage bright, and light intensity high, Offer UVB, and make sure the temps are correct. The light and temps play a huge part in their coloring.
The dragons worth depends on lots of factors such as overall health, color, size, age. If you bought a nice dragon, kept him very well and healthy, raised him close to breeding age, many times the value is higher then what you paid for him.
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