Ok, I can see the logic in the solution. I was halfway there myself, and then looked at obs20's answer and it was what I was thinking.
But at the same time, it still seems illogical to me. I mean, they can obviously SEE there are both colours there for &*&%'s sake!
It's exhausting to be in two minds. Seeing the logic and seeing the irrationality of something at the same time is where madness lies.