There's a similar one on TED I watched the other day...

Think about it... they are all in a row, one behind the other. Their crosses are marked at the back... if they know 3 would be red and 2 blue, the one that would see what their possible colour was would be in the back row... Right? If not then they do a math code to each other to figure out their colours.... I'll find the link to the YouTube TED piece that was similar.