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.
In official artwork and in the anime, Spark Kirby is depicted as having a green coloration. However, this change does not occur in any of the games.
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.
Green Kirby with dark green feet is called Kusamochi Kābī in Japan. It is a genuine official name by HAL; 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.