i think i remember seeing an interview/explanation once long ago about why blizzard doesn’t have a dye system in the game in the same way other games like GW2 do, and why having a dye system on mounts wouldn’t work.
this is because (iirc) the colors are actually “painted” direct onto the item before it is “finished”, rather than the item being “a blank slate” and then colors being baked onto it afterwards.
so you would never be able to use a dye style system in the game currently cause you will have issues sort of like why some enchant effects dont/cant override the natural graphic of some (mostly) newer weapons.
that and how else are they meant to pawn off the same horse mount 50 times as reskins?