White with a greenish tint (according to the quick search I’ve done on the interent). I wanted to say it will reflect what ever light goes into it but it was more complicated than that. If you create an infinity mirror you can see it (two mirrors aligned with each other).
I suppose my first thought was, well, what is colour?
Because colour is actually just our eye/brains’ interpretation of the wavelengths of light from the ‘visible’ range of the electromagnetic spectrum that an object reflects or scatters. Which wavelengths it scatters will depend on the material of the object at a molecular level.
The different shades/hues of colour that we see are a blend of different wavelengths of light that we more commonly associate with the colours of a rainbow – from the longest wavelengths (red) through to the shortest (violet). So a bright green blade of grass is green because it absorbs all wavelengths of visible light other than the green, which it reflects or scatters. Objects that appear black absorb all wavelengths of visible light, while white objects are often said to *reflect* all wavelengths of light, but actually they *scatter* them. This is why you cannot see your reflection in a piece of paper.
Back to the mirror then… a perfect mirror would fully reflect all wavelengths of light such that the image you see in the mirror exactly replicates what you would see if you looked directly at the object it is reflecting. Most mirrors are far from perfect though… the materials that are often used to create your every-day, affordable bathroom mirrors are actually slightly more efficient at reflecting the parts of the spectrum that the human eye interprets as green so most mirror-reflected light has a slight greenish tinge to it (although it’s mostly imperceptible).
Making mirrors is very simple. You take a flat piece of glass and apply a reflecting coating to it. This coating is nowadays non-toxic siver (hence white-metallic, metallic of silver is: blue-greenish (depends on the spectral lines)) or aluminium (silver, red-green-blue-yellowish). The reflection you see comes from polishing the coating to perfection, so no distortions occur.