I found this effect by accident and wanted to make sure I didn't forget how to do it (since that always happens. )

So you got your regular skin color at the bottom (yellowish or pinkish, doesn't matter much you can always change it later.)

Then using the skin color layer as a selection guide, make your gradients following the light source (here I used red to white) color burn and multiply both work fairly well although I've been preferring color burn lately because of how much more vibrant it looks

finally put the skin color back on top, overlay mode at a low percentage like 20% or so to balance it out.

if you want to cut back on layers and flatten them DO NOT USE THE MERGE LAYERS BUTTON THE EFFECT WILL DISAPPEAR. To combine them, hide everything other than the 3 layers, use copy merged, paste it onto a new layer and it will be good.

I basically studied a few pixiv artists on how they make art that's basically flats and yet look like they have subtle shading and found this out. It's still a pain in the ass though when you're first figuring it out, but there's something relaxing about using the gradient tool when you're not too keen to actually make a defining shade layer.

This got really long but I hope it helps I'll post the full pic of the WIP soon!