But a clear, cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more easily than they do red light.

BBC: rainbow

At sunset, the sun appears red (the blue light has been scattered out of that image, too), so the cells of the eye that detect red are overloaded and incapable of registering weaker red signals.

ECONOMIST: In a flash

And the sky is blue, it says, because the sun produces white light which is made up of all the colours of the rainbow.

BBC: rainbow

In the Bolivian Andes, ice-blue glaciers crown amphitheatres of peaks, llamas graze quietly in the afternoon sun and the evening light fades into frigid mountain nights.

BBC: In Bolivia, awe-inspiring scenes at every turn