It reflects light, so it could be white.

If you accept white as a colour, I suppose you can accept silver or silvery as a colour. And something silvery would be a good material for a mirror, no?

As for mirrors made of the likes of copper or covered with coloured glass or laquer, you might speak of coppery or other colours of mirror.

It is all a matter of how you define colour. You might elect to define a thing by the light that it reflects. In most contexts I think that would be silly, but in other contexts it could make sense.

To these answers I would add, it depends on how you define a mirror. By definition a mirror is a reflecting surface. If the  reflection was absolute, mirrors would have no colour at all. When white light strikes an object, part of the light is absorbed, this you can see as colour.

In the vacuum of space you can not see light until it strikes something. So in theory you would not be able to see a perfect reflecting surface such as a mirror. You would only see the deflection. The colour of the mirror is caused by the fact that it is imperfect.