What I do is i basically get the 3 shades of color i want and then smudge the heck out of them to make a skin tone. So its basically: Normal color, then the dark color for shadow and a lighter color for where the light is hitting. I then use the dodge and burn tool for highlighting and shadowing the areas til im happy (this is all in photoshop dont know the gimp equivalent). I've posted my wip for you to see what I mean, and I dont know if this will help with the robot coloring but it was easiest for me and as I said there is no right or wrong way. Oh and please ignore the hair I am currently working out that.