A Yellow Apple is an item in Super Paper Mario. Found on a tree in Overthere Stair, it restores 5 HP and cures poison. If the player tries feeding Peach this apple, she wakes up from the deep sleep caused by the Golden Apple and wonders what's happening. Then, suddenly, she grows a mustache. A few seconds later the effect wears off, the mustache disappears, and she goes back to sleep.
The Yellow Apple is more of an orange, but is called yellow to likely distuginush it from the Golden Apple.