Here are some screens for the issue as I was experiencing it...
Darkened portrait
Normal portrait
It seems to be pretty consistent in that it only occurs when I level. There are two places you can LvL up on the main HUD. Like in the image below If I click the arrow on the status portrait to the lefthand side of the screen then often the character's portrait will go dark. If I click the arrow on the Hotbar beneath the larger circular (current player) portrait then the portrait shows as normal afterwards. I haven't tried it from the character sheet. So far only when I do it from the smaller portrait that the darkened portrait thing happens.
I haven't experience the totally monochrome portrait issue, but it made me wonder if the lighting for the portraits is generated dynamically as like a separate aspect of the portrait, that it could maybe be down to individual graphic settings there perhaps?