I honestly think your pet store clerk is full of crap IMO.
Ive never heard of anyone ever saying that a wild caught BP is brighter because of sunlight. Not with any snake.
A snake goes in the sun for one reason, soaking up some heat. They dont benefit from UV rays like a lizard does. Thats why captive snakes do not need UV lights, they get all their nutrition from rodent prey. Most species of snakes are hidden in burrows most of day and come out closer to dusk.
Ive heard of vitamins you can sprinkle on prey before you feed, you could maybe google it? But i see no reason to really use these if your feeding appropriate sized meals frequently.
And ive never heard of anyone using it before. But you could give it a try and see if you see a difference.