Default skins use a specific indexed palette. Looks like every "key color" of each skin has a grade of 32 color tones. Palette colors from 244th to 255th are greyscale, from 192th to 223th are "bottom color" of skin, colors from 160th to 191th are "top color" of skin. Colors from 0 to 159 can contain additional colors used on skin (which should not be affected by sliders). Something like that.