Yes, PAINTBALL is a Scrabble word!

Letter values in Scrabble US: P 3, A 1, I 1, N 1, T 1, B 3, A 1, L 1, L 1.

Is it a valid word?

PAINTBALL is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

a capsule filled with water-soluble dye used as a projectile in playing the game of paintball
a game that simulates military combat; players on one team try to eliminate players on the opposing team by shooting capsules of paint at them