Except when air is red, pink, orange, grey, green, violet...

I found Walter Lewin's explanation of Rayleigh scattering to be pretty clear.

> the sky is blue because air is blue

This comes off as a smart-alecky non-answer to anyone not already familiar with what gives things colours in the first place. John Baez's answer captures the core mechanism in two sentences and one diagram, and provides more detail below for the curious:

> A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.

That is not the point. The point is that anything scatters and absorbs the light in a particular way, and we cal this phenomenon the "color" of the object. "Vegetation is green because chlorophyll is green" is deemed a correct answer. The same for the sky and for anything else.

Ref: https://www.explainxkcd.com/wiki/index.php/1818:_Rayleigh_Sc...

Substances don't have a single "color". Air, for instance, scatters blue but transmits more red. So if you look at a light source through a lot of air, it's red (such as a sunset) and if you look obliquely to the light source, it's blue.

"Vegetation is green because chlorophyll is green" is a borderline useless answer that intentionally misses the point of the question. Assuming the asker knows what chlorophyll is, all this does is shift the need for explanation one link up the chain.

Try to empathize with someone asking this type of question. Would you be satisfied with the type of answer you support? Would it trigger immediate follow-up questions that are basic rephrasings of your original question?