What can I use as background while making a hand-drawn skin?

I'm trying to make a default replacement skin using Luumia's Vanilla skin as base. My primary goal is to draw completely new face over it, leaving the body intact.

I exported LRLEs as PNGs using Skininator, but when I open the PNGs in Photoshop, faces are semi-transparent and I can't see very well what I'm doing when I'm trying to draw over them. I tried to put the default body texture template (taken from this forum) in the background and convert it to greyscale, but because the default template has shadows and highlight, I get confused and end up drawing the face either too light or too dark. Is there anything I could use as backgound, that would give me a good understanding of how intense my shadows and highlights should be? Like, a certain HEX value of grey color, or some base layer I could export from the game...

Also, if I want Maxis face details to peek through, what would be the optimal transparency for the face?

Thank you!