I’ve not done that. I’m not sure how or if it would hold up. First, the paint would have to be heated to over 300 degrees for the powder paint to melt and cover. If that did hold up, you would be binding powder paint to an overheated paint so it may not hold up.
I don’t know how a regular liquid colorant would be applied to a hard surface without mixing it with a paint substrate. Liquid colorant is supposed to be mixed with another medium like paint or plastisol