You: I bet I could make you say black! Them: Okay You: What colour is my [something red]? Them: Red You: What colour is my [something white]? Them: White You: What colour is my [something yellow]? Them: Yellow You: Hah! I made you say yellow! Them: Wasn't I supposed to say black?
I remember doing those kinda things years back in school; Also the whole 'and' trick. (Asking them to say the alphabit and then pointing out that and is a color in 'X Y and Z' or the same in 'Red white and Blue' making them look like a moron.)