Why is the evening sky blue?

It depends on which direction you are looking. Looking towards the setting Sun, you will always get at least a hint of orange / red, which is stronger when the atmosphere is full of scattering particles likedust or water droplets / thin clouds. The blue colour can be seen when you look up or away because there is still comparatively more blue light scattered downwards.
You need to be careful when assessing the colour of the sky (and many other things) because your observational powers are very much affected by context (we are very subjective about these things).
I always use the terms "reddish" and "bluish", to allow for this. This is one of those topics that are frequently discussed and there is a lot of misinformation flying around about it. Don't believe everything you read and check it all against basic theory.