Craig Mullins is talking about things I probably wouldn't understand for the life of me I guess, but from my limited ability and experience I can too tell that as long as you properly minmax the saturation and lightness of a shade, you can do pretty freaking big jumps in hue from shade to shade and it will look very well. Look at a recent edit I did for faceless' avatar for hue jumps in a single ramp. There's theory behind the hue shifts most of the time, but since pixel art allows for such minute control at any level in creating the piece, one sometimes just goes crazy and experiments with the HSL sliders on a shade, 1 bit at a time
Generally though, the theory is that you tint towards the colour of the lightsource ( blueish yellow in sunlight I guess ) and towards complementary shades in darkness. Skin strangely adds saturation in darkness in places where the skin is thin and light subsurface scatters through, but it goes towards more muted unsaturated purple darks where it is thick and oily. In pixel art, and this is more of the way I learnt to pixel from studying amiga-era artists, I usually forget proper theory when I tint art. I like to unify my palette for the piece, everything everywhere, and I like to keep the main colour of the piece as 'ambient'. It's there, but it's not THERE and THERE but not THERE. It's the general colour. Otherwise I like to use otherworldly lightsources, tints and smearings that while not realistic, suit the pixel art aesthetic I've developed over the years.
The computer is not a tool for simple reproduction of representational art like actual painting can be (amongst other things). It has it's own aesthetic. There is an aesthetic in the method. You can accentuate the aesthetic by making the method more visible in the art. Computer art, video-game art, pixel art. These things have special charges that need to be understood and to an extent exploredfor me to respect the produced art as within a new medium in itself. In that respect then, I pixel in colour schemes and stylistics I've picked up from amiga art, because clear 'reality'-based colour theory doesn't accentuate the pixel art method for me. Also, I sometimes texture using squares, or my structures ( not as in this piece ) accentuate the vertical and horisontal grids, generally giving the feel of squares, doublewide pixels, etc etc. Pixel art isn't just drawing in another way.
about green tint in flesh: I think I was trying to unify that end of the ramp with the pure gray ramp I use to tint that under the nipple for example for later when I would collapse the two ramps so as to save shades at the optimization stage. But the sickly green suits his oily flesh, no? What else would probably work would be as sick burned pink, but it would need a lot of juggling to balance it now if I changed it.
crab2selout: of course dither brushes were used. Also darken/lighten modes, some smearing around the head hmm what else...
no that's about it.