This one came up on the discussion board a few weeks ago...so I copied and pasted my old answer!
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Tape on the fingers is one of those that might be legal, or maybe not, depending on which rules you're playing under.
NCAA rule: Tape allowed on hands/fingers if judged not distractiong to batter. Tape specifically allowed if due to injury and if so tape must be a neutral color.
ASA rule: Tape allowed on hands/fingers if umpire judges it not distracting to batters.
High School rule: No tape allowed on hand/fingers that is at the contact points with the ball.
NSA rule: No tape allowed on pitching hand/fingers.
Depending on the rule set, tape on the pitching hand can range anywhere from perfectly legal to not allowed at all.