Pretty Patties are colored Krabby Patties that first appear in the episode "Patty Hype." They were extremely popular, but had the unfortunate side effect of changing the color of the fish (or their tongues) 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.
The Pretty Patties come in 22 known colors:
A dull blue-gray
Yellow with Green Spots
Green glow in the Dark
Rainbow (all the colors in up and down stripes)
Green with black and purple stripes
Role in episode
They are created by SpongeBob in trying to attract more service to the Krusty Krab after many days without business. They are laughed off by Mr. Krabs and Squidward when SpongeBob introduced them, but after he set up his own stand to prove it could work, it was an instant hit with the town. They are loved by all Bikini Bottomites and the stand easily outsells the Krusty Krab, which was seeing fewer customers for a while. However, before long the colored patties turned their skin into an assortment of colors, and even cause three citizens' tongues to change different colors: one glow in the dark, one yellow, and one orange with green dots. Everyone hated them afterward, and they haven't been seen or sold since.
The venue in which Pretty Patties are sold is 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 money away to customers. The stand is later taken over by Mr. Krabs, and the Krusty Krab key is given to SpongeBob. After the episode, the stand no longer exists and the customers demanded all of their money back.
The last Pretty Patties customer was Frank (citizen), with the glow-in-the-dark tongue.
If Pretty Patties used the same recipe (with exception of the food paint) for Krabby Patties, SpongeBob would owe Mr. Krabs royalties fees.
When SpongeBob revealed the Pretty Patties to Squidward and Mr. Krabs, he says, "Available in 6 designer colors." However, they are seen in approximately 22 different colors. It is possible that he added the colors as they got more popular.