How does matter (on an atomic scale) get its color?
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For low concentration atoms and molecules (such as gas, ions in solution or impurity ions in a solid), the electrons can only move between a few filled up orbitals and the empty orbitals. Each move corresponds to a precise energy and wavelength (simply put, a portion of the light that corresponds to a color). The combination of absorbed wavelengths form the color.
For high concentration atoms and molecules (such as the main components of a solid or liquid), the electrons can move between filled up, semi-filled and empty bands. Any move corresponds to a wavelength band (simply put, a portion of the light that corresponds to a band of color in the spectrum, violet to red passing by blue, cyan, green, yellow, orange). The moves inside the semi-filled band correspond to reflection if there is a semi-filled band (only metals have semi-filled bands). The reflected and absorbed wavelengths form the color.
When light is absorbed and an electron moves from one orbital to another, or from a band to another, the electron will move down afterwards by transferring the energy to heat, or sometimes by emitting light.
When light is reflected by having an electron move inside a semi-filled band, there’s some chance the electron loses its energy and doesn’t reflect the light. Then, the light is absorbed. The more resistance to electricity the material has the more likely it is that it absorbs light. That’s why high conductivity metals like silver have the highest reflectance.
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