A mummy with green skin he was, and a mummy with green skin he ever shall be. We can't see much of DB's skin - just his lower lip and his eyelids - but what we can see is a minty green; the rest is black. There are short Madballs cartoons on YouTube; I haven't watched any, but if Dust Brain doesn't sound like Peter Lorre in them, then what even is the point of life having evolved on our planet? He's got the drowsy eyes and the squint, and even the shape of his mouth feels like a Looney Tunes caricature of the man. The X-shaped pupil makes sense here, since Dust Brain is dead, and the whites of his eyes are actually tan. A little bit of green drool dribbles out of the corner of his mouth, and there's a pink-orange scarab tied into the bandages on the back of his head.
Series 2 of the blind bags featured "Mix-Ups," re-releases from Series 1 with new colorschemes. Dust Brain got redone all in shades of blue, dark for the bandages and light for the skin. Why would a mummy be blue? Because reasons.