So, according to the MC wiki: https://minecraft.gamepedia.com/Formatting_codes
Signs can use colors when you use the § simbol with a number
But right now, quark won't let me!
Nor actually copy-pasting the symbol will work.

Maybe adding a button right next to the "Clear" and "Shift" ones that inserts the "§"?

Thanks, if there's actually a way to do it, let me know, as I couldn't find it.

Won't happen.