Many Bettas have the "jumping gene", and demonstrate "marbling". They will change colors, sometimes dramatically, and can do it quickly (over a week) or slowly (over months). My favorite that I had was a very large Plaket (short fins) that was orange with red fins when I bought him, and had turned all blue and red by two weeks later. After that the colors just slowly migrated around his body for the next two years. As long as its just color changes, and not a fungus or skin condition, I don't think there's anything to worry about. But I am new here, so others might have more to add.