He is, IMO, dark green. I do see some of the blochiness/uneven green color on his chest and belly, but that blochiness is much more visible when I took a pic.
The blueish color you see is dark green in reality.
What is his color called? or is it an imperfection in his coloring?
He looks like a yellow face type II sky blue, I can't tell about his back because I can't see it. He looks green because the yellow blends throughout the body in the type II mutation, so it combines the blue and yellow to make a beautiful green
Considering how bright yellow his face is and how green his body is, I would say he's a goldenface. Goldenface and yellowface budgies are actually blue budgies with yellow that spreads into the body colours from the face. The degree of spreading is often slightly different depending in the individual and whether they have one or two factors, so it's not too strange to see a blotchy bird. Colours sometimes even out more after the first molt as well. I hope I helped, good luck!