genuinely if you add me on discord i will absolutely walk you through it ! i can answer here briefly but i can provide visual examples on there !! x.vk

With darker colour palettes I follow the same technique as with a lighter palette (using hue shifting), but up the saturation so you can make out the colors without it looking muddy / grey. I typically make it in a normal saturated style before using a tone curve to make the colors darker / add an overlay on top. I wouldn't say it's necessary to use saturated colors but I personally prefer it! You can always balance the values a little by using brighter colors in places (such as a brighter false light source) or like the pupils to suggest a glow :) Even with a dark tint it's good to maintain differing values even if it's not as dramatic + can help something look more appealing. If it helps, you can make a skin + to check if the colors work together, brighten the image on your phone/program to see if they blend when not under a dark tint :3. (again can provide visual examples for all)

For white hair I shade it with literally any other color lol ,, if you shade towards a specific color (e.g. the darker parts of the pony appear more blue), carry on with that. I typically don't choose pure white and instead a slightly tinted color (e.g. slightly yellow, and shade it like desaturated blonde hair ..) (desaturated yellow - orange - red - pink - purple - blue) but you can do whatever suits the skin the most. Essentially just shade it normally but move the slider around to different colors :3