I am a hobbyist who does a lot of Cerakote.
It does not matter at all what the underlying color is.
When I do a multicolor camo job, I usually do the lightest color first.
But it does not really make a difference, the white will cover even the red underlying color.
Cerakote is super easy to do yourself. You need an air compressor, water filter, and a cheap spray brush like a badger. Practice on a metal tumbler or ammo can and then you are good to go on your gun.