Alderney from 30,000 feet. Air is blue whether seen looking up through it, or down through it.*
Things which are totally opaque, and don’t have a mirror-like surface, have only one colour: it’s the colour of light that they scatter. Any other light is absorbed.
Things which are totally opaque but have a mirror-like surface, have two colours. Some of the incident light is absorbed, some is scattered, and some is reflected specularly – that is, mirror-fashion. In general, the colours of the scattered light and the reflected light are different – in total, less than the incident light by the amount absorbed. So for instance, you could have something that reflected red light, scattered green light, and absorbed blue – although of course in general it’s more a matter of different percentages of each than all or nothing.
Transparent things generally absorb some, scatter some, reflect some, and transmit some. Many transparent materials scatter and transmit more or less the same wavelengths, while absorbing most of the rest and reflecting only a small percentage of everything almost equally. This means that bright objects seen through that material are coloured according to the transmitted light.
Reflections are not (usually) an issue with gases, as there’s no surface to reflect from, and absorption is rarely a significant issue (for wavelengths we can see). This means that the only variation possible is the percentage of each wavelength that’s transmitted or scattered. Air transmits almost the whole of every visible frequency – but does scatter a small percentage of all of them, but more at the blue end of the spectrum than the red. This means that a bright object seen through a sufficiently great distance of air is reddened – and that the light removed from this direct path, which is bluish, is scattered in all directions. The sun is so bright that the scattered light in all directions overwhelms starlight, for example, and if you’re looking at some other object (much less bright than the sun) through a great distance of sun-illuminated air, then the bluish scattered light is added to the light from what you’re actually looking at through the air.
(All this is somewhat complicated by the fact that the proportions of colour being absorbed, scattered, reflected or transmitted can depend on the angle at which the light hits the surfaces of solids or liquids, or even the angle relative to crystal axes in crystalline solids; and the proportions of colours scattered in different directions can also vary! With many materials they don’t vary much if at all, but in some cases they do.)
* Incidentally, this is also why the planet looks blue from space – not because it’s mostly ocean, a fact even astronauts generally get wrong. Water isn’t blue; the sea only looks blue because it’s reflecting blue sky. When the sky isn’t blue, the sea isn’t blue either, it’s grey or greenish-grey, a fact you can easily observe for yourself.