As of patch 7, this still occurs. I did happen to notice (on my mage) that the change makes it look as though she's wearing her unarmored leggings. I'd say it looks identical, even. Amusingly enough, boots equipped as leg armor are subject to a color change as well.
Given the way the gore effect is implemented and the general fidelity of the game's graphics, I don't think it's an intended "dirt effect." I feel that it might be an unintended superimposition of the gamma/colors of the character's pajamas onto whatever he or she is wearing. To support that, I think some other people in this thread said that using white clothing does not encounter this issue; maybe that's because the white mesh/texture/whatever can't be lightened any further.
I really do hope that this is addressed in some way soon, though. The armor in this game is very fun to see and it's a bit of a heart-breaker to see such rich colors suddenly interrupted by a dull tan.