This is something I have always wondered, but Elise at IFLS and Michael at Vsauce state it better than I.
Everyone is told that the sky is blue, so we look at the sky and see blue. But what if what I perceive as sky blue is the same color that you perceive as sea green? Or cotton-candy pink? We don't know. We are both looking at a blue sky but may be seeing marginally different (or even radically different) colors.