Sorry for the lack of WIPs on this pic. Here are jpg’s of all the steps (with not-so-very-clear notes).
All the work was done in GIMP.
Wait what? A grayscale shade layer??? What layer option do you use though, let alone change the color successfully?
This is a really effective way to color, actually. I do it all the time. When you make shade layer, make sure you’re ONLY shading SHADOWS and basic diffusion — NOT object/material color values (like, just because the socks should be a dark color, you’re going to completely ignore that in the shading layer.) There are two ways to mix the shading afterward. You can place the shading layer over the flats layer then set the shading to Multiply, OR put the flats over the shading and set the flats to “Color” blending. Then, you just paint some small variances in hue to whichever layer you’ve set blending mode to.
This is a great way to color because it eliminates the need to mix new colors as you pass from one flat to another. It gets less of a painterly look, but in this type of art you’re not going for that anyway. Excellent for comics.