Completing certain tasks get you different basketballs.

Effect - Code

All Silver Ball - 2000 points on profile
Black and Red Ball - Play Rainbow ship on exhibition without coins and win
Blue and Silver Ball - Complete Training Mode
Cheep-Cheep Ball - Get Gold in Hard Mushroom Cup
Dice Ball - Get Gold in Hard Flower Cup
Gold Ball - Collect 5000 points in profile
Goomba Ball - Get Gold in Hard Star Cup
Pumpkin ball - Lose 5 times in wireless exhibition
Purple and Orange - Play Bowsers Castle in exhibition mode for the first time
Rainbow ball - Win 5 times in wireless exhibition
Red white and blue ball - On any tourney match,win by ONLY using charged shots
Soccer Ball - Get Silver In Hard Mushroom Cup
Spiked ball - Get a gold trophy in Star Cup
Star Ball - Use all special moves in the Star Cup
Watermelon - Get Gold in Flower Cup
Yellow and Green - Play Daisy's Garden in exhibition mode
Yoshi Egg - Get Gold in Mushroom cup