Unlock the following bike skins by performing the corresponding tasks:

BIKE SKIN - HOW TO UNLOCK
Electric Blue - Get 100% S ranks in the Platinum Cup
Forest Green - Get 100% S ranks in the Bronze Cup
Lemon Yellow - Get 100% S ranks in the Bronze Cup
Plum Crazy - Get 100% S ranks in the Silver Cup
Sea Green - Get 100% S ranks in the Gold Cup
Speed Green - Get 100% S ranks in the Silver Cup
Sunset Orange - Get 100% S ranks in the Gold Cup
Void Black - Get 100% S ranks in the Platinum Cup
Skull - Get all 100% S ranks in all four cups