How does grey occur in elemental materials such as metals? I believe that grey arises from the simultaneous reflection and absorption of all colors of the spectrum (in different atoms of course), as mixing opposite colors on the spectrum would do. How would this occur in a one-element material? How does this occur on an atomic level?
A clean un-oxidized metal surface is not usually grey but rather reflective like a mirror. In fact, mirrors are constructed by coating a sheet of glass with a thin layer of metal atoms. The grey color of a slightly oxidized surface, which includes almost metal surfaces you encounter in daily life, look grey because some of the ambient white light is absorbed.
How would this occur in a one-element material? How does this occur on an atomic level?
Metals have relatively mobile electrons. Therefore, when an electromagnetic wave comes in, the electrons can move around easily under the influence of that wave's electric field. That means that an incoming wave causes the electrons to oscillate at the same frequency as the wave itself. Therefore, the electrons in the metal surface emit radiation at exactly the same frequency as the incoming wave. In other words, the light is reflected at the same color as it had before.