Why the sky is blue

This morning driving was a pretty moody dark cloudy day (by the way -  the colors of the cars are becoming more like the colors of refrigerators, gray, silver, whites, given that they have the higher resale value, thus driving a recursive share growth increase among car colors), and the blue sky was hidden by dark clouds, no green around in 101, no yellow sun, my whole view was rather gray-ish and that made me sad.

That took me down the path of trying to understand why blue. We know why the sky is blue, but what I was interested is why we can see blue? Is it because we want to be able to see better the sky? That seems illogical. For that matter why do we see green or yellow or red. Are they dominant frequencies and thats how the eye evolved its frequence sensing to be able to capture the maximum amount of frequency variance available?

I think not.

Here is my simple way to find out why we see the colors that we can see (again  following this methodology of assuming no computer internet wikipedia available - just our logic and our memories). Imagine everything is black and white. Imagine we are   10 millions years back in some anthrpoid form (we've heard dogs see colors different - so I am assuming we do not need to go back 100M years to mammal common ancestors).

So we are a chimp like creature that sees everything in black and white. In what way that "sucks"?Clearly we can tell the sky - even if we are hanging by our tail  upside down - it is extremely easy to tell in black and white picture.  But we can't tell the parts of the tree that are soft and green thus edible from those that are more hard and brown. That is a massive advantage when trying to pick the soft edible parts of a plant. Ok that explains green. We see green because green is the leaf color (ie its the favorite color for clorophyle (why that is so is another question))

How about red. Why would we see red? Red seems a not very natural color. It is there in some fruits but we can make the case that the fruits like red apples and berries learned that the eye can see red and become red to be more easily discovered and eaten. Maybe red - goes further back. Maybe we see red because of our blood. Red liquid on a rock, or on an animal means that there is an injured mammal. It could mean food, it could alert to an injury. So we see red because it is the blood color (ie which happens to be red because of the red cells which happen to be red because of the iron in them?))

How about blue, our first question? I had a harder time with that. The best answer I could come up was that due to our possibly aquatic past, being able to tell water that is clean from water that is not is important - and clean water has a fine blue tint (even in a lake/river). So we see blue because of the water not the sky.