Sorry for the late reply! This is what my artist said in response to your question:
"i dont generally use any pre-established color palettes, though for mr. guide his hair color is so specific i sometimes have to reach for the good ol' color picker tool of his initial sprite. besides that i just usually pick warm desaturated colors, + his hair color is so specific because it reminds me of many media's respective komaedas, lol. i feel like the moment a character has that grey-green-ish tint to their hair you can tell they are NOT normal"