"Reason" is a matter of one's choice of level of abstraction.
The reason a banana peel is yellow is that in a lit environment, it makes yellow light hit your eye.
The reason air is blue, is that in a lit environment, it makes blue light hit your eye.
Obviously one can go further to deeper reasons. Go deep enough and no different colors have the same reason. But you not understanding the level of abstraction in use doesn't make me wrong.
The sky is not blue for the same reasons that a banana is yellow.
But the reasons behind color in both cases is interesting regardless.