It doesn't have a color. It's a piece of transparent glass that is tilted in a way that you can see the reflection of light off of it in a form that is an exact copy or "mirror" image of what is in front of it.
This may be a trick question. By definition a mirror is a surface that reflects light so you could say it is the color of that color. If the mirror is aluminum then perhaps gray. It is really the color it reflects accented by the color it does not reflect.
Most mirrors are backed with a foil of a silver tone. So, you could say either that a mirror is silver, or that it's a metallic grey. Some "designer" mirrors can be gold or bronze colors. I've even seen a mirror with a black backing--it didn't give a great reflection, but it was very attractive.
a glass can be said colorless if it is fully transparent but if we are talking about mirrors then they are also colorless but they are painted with a shiny color ( generally silver ) on their back side so that it reflects light and we can see our face. :)
Mirrors are made by taking a smooth piece of glass and putting silver chrome paint over it. It has to be completely smooth obviously. The chrome is reflective though so it makes it so you can see yourself and other things in it.