This is a test of commonsense. Complete the next sentence:

How to choose a goalie pinnie
Choose a colour other than your team's.
You may think this step is crazy, but it's true. If you're wearing a blue goalie pinnie, and your team's colour is blue, you won't be allowed to wear that pinnie in play.
This is because the referee can't easily tell who can pick up the ball, and neither can the other team. This gives you an unfair advantage, which is why it is illegal.