Originally Posted by pdurrant
Do the thought experiment - pretend you're the first prisoner to arrive, and you're given a green mark (although you don't know it's green). Subsequently one more green and two blue prisoners arrive. Every prisoner can now see at least one prisoner of each colour. So every prisoner knows that the prison contains prisoners of both colours. Can any of them conclude anything about their own colour from this? I think you will see that they cannot, especially if you try to consider the situation from the viewpoint of any one of the prisoners.
You're right, the governor is letting them know that everyone now knows there are two colors represented. But I think it only works when you have 2 of each color.
As soon as you have 3 of each color, you can be assured that everyone can see at least one of each color.
Of course I could be way wrong