The buttons on the whistle are white and red. What if one of the teams is wearing blue?
BTW - I use a chapstick in my pocket to keep track of possession. In kids rec games, it's imperative I do this because the parent volunteers at the table usually don't understand the possession protocol. The chapstick seems to be just the right size and weight. I used to use a Hickory Farms beefstick, but I'd get weird looks from some women coaches when I'd move it from side to side.