Comment by adrian_b

You have a kind of cone with maximum sensitivity in green, because that is what plants reflect, not the other way around.

You have another kind of cone with maximum sensitivity in blue, because that is what the clear sky provides.

So a mammal looking upwards (or forward for a tree-dwelling animal) can distinguish easily leaves from sky.

Monkeys and us have another kind of cone with maximum sensitivity in red for 2 reasons, blood is red and many mature fruits have reddish colors (including orange and purple).

The redness of blood matters not only for noticing wounds, but also for detecting emotions on the faces of mates or adversaries (due to increased blood flow; for some monkeys the emotions may be seen not only on faces, but also on other hairless areas, e.g. buttocks).