I think we can simplify the answer to this question for most audience and say "the air is blue".
If they say, the air appears to be clear when I stare at something other than sky, the answer is you need more of air to be able to see its blue-ness, in much the same way that a small amount of murky water in your palm appears clear, but a lot of it does not.
If they ask, why don't I see that blue-ness at dawn or dusk, the answer is that the light source is at a different angle. The color of most objects changes when the light source is at a flat angle. And sun lights hits at a flat angle at dawn and dusk.
If they ask, what exactly is the inside phenomenon to see the sky color to be blue, then explanations like this blog are relevant.
If they ask, what exactly is a color, the answer is that it is a fiction made up by our brain.
As confusion elsewhere on this page illustrates, one also needs to clarify absorption. "It's just blue" sky and "it's just blue" stained-glass have quite different behavior. Both side scatter some blue, but while one mostly transmits the rest, the other mostly absorbs the rest, for very different experiences peering through it.
So perhaps "clear with a blue tint"?
Yes, I came here to say this. The whole topic drives me crazy. Air is just blue. Everything is a color because of some physics reason. Some birds have blue wings due to microscopic structures and how light interacts with them, rather than pigment.
If you took a large column of air into space and shined white light through it, it would be blue.
No, it would look red. The weird thing about air is that it's not reflection or absorption that gives the color, but scattering, and that means the color is strongly dependent on what direction you are looking at it from in a way that most transparent mediums aren't.
Ok, so the air would be red from one angle, blue from another. In each case, that is what color the air “really” is, in the same sense that a butterfly’s wings are blue (but not from every angle)