What would you say the colour of a mirror is?

It's a good question.

I don't want to be too clever with this answer, but I can't think of a better one, or one which is truer.
Interestingly Andy Warhol loved the idea of reflection and he often used silver to embody this, including on the walls of his studio space The Factory.
There is a great Andy Warhol documentary on BBC iplayer now, worth watching:

https://www.bbc.co.uk/iplayer/episodes/p0b5mp52/andy-warhols-america