Why Isn't the Sky Green?

My fiancee and I were talking about kids tonight, and it got me thinking about how I would handle being a dad. One of the bigger concerns I have revolves around what will happen when our future kid gets to the "why?" phase. I wouldn't be able to fully express the idea of Toyota's Five Whys, so that's not going to stop it. Ultimately, I want to encourage asking why, but I also want to encourage asking it with purpose.

This made me think of the question "Why is the sky blue?" If my future kid--and he or she will be a Future Kid--were to ask me that, my response would be, "Well, the sky isn't blue all the time." Sometimes the sky is red or orange or yellow or purple, indigo, and even black. But it's never green.

So why isn't the sky green?

This absence of green in the sky makes me wonder if this is an explanation for plants being green. Could it be that photosynthesis evolved to take advantage of the one color that the sky doesn't filter?

This is the kind of thought I want to encourage in my kid. But it's also the kind of thinking I like to encourage in people. By refining our fundamental questions we often arrive at unexpected truths or realizations. Unfortunately, we too often stop at asking why once, or we repeatedly ask why without direction.

Why is that?