I always got the vibe that Trap Jaw was the same race as Skeletor* (at least the same race he was back when he was Keldor), so in designing this, I figured the logical way for his face to get green was to peel off his skin via severe scarring.
If you had the original Skeletor figure from the old MotU toyline, he had green around the edges of his yellow skull. I like to think that the race Keldor and Kronos/Trap Jaw are have blue skin, green muscles and blood, and yellow bones.
I have a few ideas of where to go next, so expect more of these in the days to come. (TJ here was done as a cool down pic to cap off my night.)
* A friend has informed me that they are the same race - a people known as the Gar.