I've discovered this days that people doing pixel art uses to make fixed palettes. Picking 10-20 colors and drawing using only these; and thats how they achieve to create personal atmospheres and ambients in their drawings.
I thought in making one for the book myself: I think that a homogeneization of the color on the whole art will make it have a more consistent look. These are some tests that I actually liked
Randall flagg; the walking dude, from Stephen King's The Stand
A newer, more "serious" version for the huldra
A barman; which reminds me of someone but I can't grasp who right now.
The fourth (and I think it's the final) portrait of Captain Ferard
And the color fixed Laura Bow; cuter than ever