The sky is blue.

I don't even need to go in detail about this one. If you look outside and look up into the sky, you will see what I mean. There is no possible way that the sky could be any other color than blue (except maybe at night when it is dark outside or white if you count the clouds).

It depends on what you mean by 'is'. Blue is the color that is reflected. The sky could be accurately called all colors BUT blue.

Then again at sunset it could be many colors. Or if one looks up and its over cast in a city, the same.