For my art, I only use
ink for the outlines and for coloring, I use darken only at 100% opacity (because it wont go over the black).
For shading, try using darken only w/ black and 25% opacity
For lighting, try using soft lighting w/ white and 25% opacity.
I use GIMP as well, so I know what's up bruh...
For coloring (or recoloring), you can use color at 100% opacity using your color of choice. It can even change colors (like green to red in the head above) and yeh.
I don't have any robot heads 4 u doh...