First it is not a paint, it is a carotenoid if you want to be scientifically correct. However in not manner is it a paint.
Color in salmon is also based on species, a lot of farm salmon don't use species that turn a less red shade after eating the appropriate carotenoids. In addition depending on the species you have a percentage of wild fish that not affected by carotenoids. Finally the time of the year the wild fish was harvested will dictate the color.
As for marketing that info on the red color is really old. For the past couple of years the white salmon have been the most prized. You will find it marketed as ivory or other names.