mirrors are white.

This is something that many people don't often realize. What color is white gold? "Silver", which is white with lots of smooth surfaces.

White light has three colors: red, green and blue. If you shine a white light onto something, it reflects the true color. Only white light reflects back red, green and blue light. A mirror reflects back red, green and blue. Thus, it is white. Paper doesn't reflect an image because of its coarse surface.