Most of the time people assume the change of color in a Betta is negative, but in this case it's a marbling gene that causes the fish to change colors. November 8th when I purchased Isaiah, he was primarily white. I posted a photo of him back then showing at much.
It's also noteworthy that when the fish is in the store it's a lot more stressed and not fed well and in a tiny bit of water. Once you bring home your Betta and give it proper care, the true colors will begin to shine.