It depends on the mirror itself, the quality of the glass/plastic and whatever the reflective surface behind the glass is, now, Usually, Aluminum, which is silver/white in color, and there are also other types of mirrors that use other reflective agents... but it used to be Mercury, which is called Quicksilver for a reason.
So now to answer the question:
What Color is a Mirror?
Unless it is a 'funhouse mirror' with an odd tint so the person looking into is suddenly purple or green.....
Technically, everything reflects all colors of lights - just some stronger than others.
If you have a theoretical absolute mirror, it would reflect light of all colors equally. So mirror is all colors?
But. If you have light that consists of colors/wavelengths, we consider it white. So I'd say a mirror as general concept is white.
For actual existing mirrors, I agree with Doc: depends on what it's made of.