A Pretty Patty is a colored Krabby Patty that appears in the episode "Patty Hype" as well as the online game Which Krabby Patty Are You? They were extremely popular, but had the unfortunate side effect of changing the color of the people who ate them to the color of the patty.
They look like normal Krabby Patties, but are differently colored. They still have the same ingredients as the original Krabby Patties, however.
They are loved by all the Bikini Bottomites and the stand easily outsells the Krusty Krab, which has been seeing fewer customers for a while. However, before long the colored patties turn the customers' skin into an assortment of colors, and even cause three citizens' tongues to change different colors. As a result, everyone hates them afterward and the the episode ends with Mr. Krabs getting chased by an angry mob.
Pretty Patties are sold at a stand created by SpongeBob and Patrick. There is a stand to sell the patties, and right next to it is a stand that gives free money away to customers, much to the horror of Mr. Krabs.
The stand is later taken over by Mr. Krabs, and the Krusty Krab key is given to SpongeBob.
Soon afterwards, the customers demand a refund after discovering that Pretty Patties turn them into the color of the Pretty Patty that they ate.