some supplements can yes but even though you are enhancing the color of the gecko when you quit using the supplement or whatever they gecko will lose its color. i believe if you are doing this for yourself and its a good supplement, that is fine but if you are doing it to make your geckos look better to sell them then that is wrong.
At the same time your gecko's are what they eat and what you feed what they are eating. The common term of "gutloading" apply's to loading the insect up with the good stuff before feeding. But to add an addative to add color would be along the lines of enhancing the animal to sell. But to plain feed your feeders good healthy food to increase the health of the gecko eating it should be a no brainer. I gut load everything I feed to my animals.
I've heard that paprika helps enhance colors, (was it reds, in particular?), and I know that some people include it in their crickets' gutloads. (The situations I'm thinking of were for frogs, though.) I think bee pollen was also in the gutload mix for a similar reason.
I have read that anything with a natural amount of orange in it has a genetic make up of beta ceratine in it. but adding it as a supplement to your reptiles is different