Xfinity Won’t Exchange My Phone

Today I purchased a new Iphone from a Xfinity Store. Went back the same day wanting to exchange the phone for a different color and was told I was unable to because I paid for the phone in full. They can only make exchanges if I was paying it on a monthly basis. As usual no one in store was unable to give me a reason why or helped find a solution. Has anyone have a similar situation happen or found a solution to this?