Modern mirrors are made by silvering, or spraying a thin layer of silver or aluminum onto the back of a sheet of glass. The silica glass substrate reflects a bit more green light than other wavelengths, giving the reflected mirror image a greenish hue.
No, really.
https://www.zmescience.com/feature-post/natural-sciences/physics-articles/matter-and-energy/what-color-is-a-mirror-its-not-a-trick-question/