Correct me if I'm wrong, there are only 9 possibilities?
Problem: 3 colours available for football strips (Top & Shorts)
Rules 1)you can have different colours on the top and the bottom
2)you can have the same colours on the top and the bottom.
So there are only 9 possibilities? E.g.
Top colour Y Y Y B B B R R R
Shorts colour Y B R Y B R Y B R