Dependent on which fish you're talking about, genetics can play into coloration. Some fish (like Ruben Reds) were line bred for color.
Food can also be a factor with some fish, and there are some brands of food that will make a fishes color pop.
Finally, some fish have been dosed with hormones. This is more prevalent in Asia, but does happen in the US.
I would also not be surprised if Photoshop was a factor with some unscrupulous sellers. The best way to ferret that out might be to call the seller and ask them to text you a pic of their current stock or buy the fish in person.