Though is some cases it is a very interesting question, like why gold and copper the color they are instead of being boring and silvery like all the other metals?

The transition metals are colourful because their outer shell of electrons (d-block) are about visible-light sized.

Looking at a periodic table makes this pattern kinda clear.

Color can be due to reflection, emission, filtering.

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.

"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.

Not really. If the explanation was "air is blue" then the naive expectation would be that sun would appear blue against blackish background, basically the image of sun is being filtered through the atmosphere; if sun is white and air is blue then white filtered through blue should be blue? But sun appears yellowish against blue background. So clearly something different is going on.

But a banana is yellow for a very different reason (and a much easier to explain reason) than why the sky is blue. And air isn’t blue, because it’s red at the end of the day?

Why is a banana yellow?