Why are juvenile blue tits green?

I could not give you the scientific explanation but what I know is that when you mix blue and yellow, which are the colours of an adult blue tit, you get green. So, somehow, the bird's body is not mature enough to process the colours in its body hence unable to separate blue from yellow. What do you think? Is my theory any good? You can also watch my video of a juvenile blue tit on http://www.youtube.com/watch?v=AmhBhL-y1ic. In the meantime, is are a few still images taken from it.