We didn't do it on-board, but we have at Aulani. I assume it's the same... the animation cell itself is a clear plastic sheet (the size of a piece of paper) with black outlines of a character. Maybe you've gotten one of the souvenir animation cells in the mail after a parks visit? If yes, it's that kind of plastic sheet. Since it's just printed with black outlines, you paint it, however you want. Takes a while to dry, which is sort of a pain when you're on vacation, but it's a cute souvenir. It's possible they strategically plan that for port days b/c there is a limit to how many people they can have paint at one time. At Aulani, you had to sign up for a session for that very reason.