When you take a look at the spectrum of light reflected by a mirror, you will find that it best reflects light in the 510 nanometer range of the visible spectrum, which we percieve as green.
This is easily observable when you place two mirrors face to face, and look down the endless tunnel of reflection.
Notice the tinge of green? With each reflection, a little bit of light is lost, but least of all green. Thats why the reflection down the tunnel is dimmer and greener. Therefore, a mirror is a tiny bit green.
I dont know about you, but I find that to be awesome....