Light and Mirrors

A surface that reflects or absorbs all colors equally is called grey. Black and white are just the different ends of grey.
A mirror (ideally) reflects all wavelengths equally.

There is a similair definition of color - that a black object emits light only depending on it's temperature, the color of the object doesn't matter. Slightly confusingly the sun is a black body - it's color is only due to the temperature - all the different colored stars in the sky are simply different temperatures.