Swapfell is another one of those substitute AUs for Underfell. It is sometimes known as Fellswap. It is a Underfell and Underswap combo or fusion.
As Swapfell is Fell-ified version of Underswap, there are several changes with the characters. They also commonly wear purple, since in Underfell, most character designs include the color red and in Underswap blue is the most commonly used color (in which combining the two make purple) . The Underswap characters' personalities are the opposite. For example: Underswap Sans is kind and joyful, while Swapfell Sans is very ignorant and mean. Swapfell Sans is very much like a brat. The characters behave like their Underswap counterparts, but with opposite personalities.