Quote Originally Posted by Mr Beer View Post
The feathers could be dull on the outside and vibrantly coloured on the inside, only showing their feathery glory when raised erect. This could occur during mating displays (because that's what they are there for) and when it attacks adventurers (because it looks cool and once it lunges it doesn't need to be hidden).
They could also be dull colored from the front and bright when seen from behind, because a T. rex doesn't need to run and hide from anything.

It's just not the same when it's attacking you.