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

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

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