Finn's sweaters

Finn is notable for possessing two sweaters, each of which appears to have a different function based upon how Finn uses them.

Yellow Sweater
The Yellow Sweater is a sweater Finn usually wears whenever he goes into the Ice Kingdom in order to deal with the Ice King. He originally wore this sweater in the original Adventure Time short and has since notably worn it in "Prisoners of Love" and "The Chamber of Frozen Blades" (where Finn has to be reminded to wear it by Jake).

Pink Sweater
The Pink Sweater is a special sweater knit by Princess Bubblegum for Finn to use during his encounter with the Lich in "Mortal Folly". It is a light pink sweater with a heart embroidered into it. Initially Finn notices how warm the sweater is after putting it on, but it becomes crucial in being a major weapon in taking The Lich down due to possessing the power of "liking someone a lot" (Finn's concept of "love"). Finn rips The Lich's eyes out with it and defeats him. In "Mortal Recoil", Finn tries to use it again when the Lich possesses Princess Bubblegum, but unlike the previous battle, it fails.

Trivia

 * Fionna, Finn's gender-swapped version, also wears a yellow sweater when she goes to the Ice Kingdom. The only difference is that she has a skirt rather than pants.