I just discovered a G'MIC filter that might prove very useful to me. Visually and generally speaking, shadows tend to be complementary colour to the light that is casting them. It's actually an optical illusion, but because we see it, we paint it. The Vintage Style filter in the colour section of G'MIC seems to be a good way of establishing these colours when used on a duplicate overlay layer.

Actually, it doesn't work too well with colour. Here I used a lighten only layer for the shadows and a darken only layer for the light with reduced opacity. Excuse the sloppy colouring - I threw this togethter to demonstrate. I'm not even at the colouring stage yet as I spend too much time on Gimp Chat!

Very nice work erisian, your drawings have beautiful skin tones.

Thank you Molly.

Think I'm going to try turning the whole image blue before adding the colour overlays and at the end, add a darken only layer with the direct light colour.