Green Kirby is a playable character in Kirby Air Ride, and appears as player four in some Kirby games, such as Kirby: Nightmare in Dream Land, Kirby & The Amazing Mirror, and Kirby's Return to Dream Land. In other games, he simply serves as a palette swap for regular Kirby, or just a CPU controlled challenger.
The main difference between Green Kirby and Kirby is that he has a differing color scheme. His main body is green, but his feet are either orange or dark green depending on appearance. In sprite-based games and games in the Super Smash Bros. series after Super Smash Bros. Melee, they are orange; in 3D games excluding those in the Super Smash Bros. series after Super Smash Bros. Melee, they are dark green.
Green Kirby with dark green feet is called Kusamochi Kābī in Japan. It is a genuine official name by HAL Laboratory; they, or Masahiro Sakurai himself, call him this name on the Japanese official site of Super Smash Bros., 'Sumabura Ken'. A kusamochi is a Japanese food; it is a grass rice cake.
Green Spray Paint can be found as a palette swap in Kirby & The Amazing Mirror and Kirby: Squeak Squad. Variants called Emerald and Citrus can also be found. Emerald Kirby is shown with a mint green body and orange feet. Citrus Kirby is shown with a bright yellow-green body and green feet. The former is found in both games; the latter is only found in Kirby: Squeak Squad.
While Green Kirby himself does not appear in the anime, in Kirby Comes to Cappy Town, Kirby turned green when he was in a weakened state. In The Fofa Factor, when Slice n' Splice spilts Kirby into two, one of the Kirbys, the "male" one, is green. The last appearance of Kirby turning green in the anime was in Air-Ride-in-Style - Part I, when the Warpstar was damaged and turning green for a few moments also made him green. In official artwork and in Kirby: Right Back at Ya!, Spark Kirby is depicted as having a green coloration, however, this change does not occur in any of the games.