Where does a mineral get its color?
The color any object appears to be is actually the color most readily reflected away from it.
The color of a mineral is is commonly due to the presence of trace elements and/or defects in the crystal structure and can be described in three broad categories.
Crystal Field Transition: Electrons in the d orbital of transition elements (a.ka. chromophore elements) leap to higher energy levels when a light is shone upon them by absorbing some of the light at the wavelength corresponding to the energy difference between the levels. For example, if you shine a flashlight on peridot it will absorb the red wavelengths of light and appear green.
Molecular Orbital Transition (i.e. intervalence charge transfer): Minerals with adjacent trace cations (positively charged ions) will have non-localized electrons that will jump between them, absorbing or emitting light with each transition. An example of this can be observed by sapphire (Al2O3), a variety of the mineral corundum, which has trace amounts of Fe+2 and Ti+4 . The transition of the electrons as they move between these two cations will emit blue light (because they will absorb red through yellow light).
Color Centers: Defects (vacancies from missing ions) in the crystal structure are filled with electrons to balance the missing charge. These electrons will absorb certain wavelengths of light. These defects can be produced/multiplied by the bombardment of highly energetic radiation. For example, the purple color of amethyst (SiO2) can be darkened through irradiation.
The yellowish staining on the surface of Europa is believed to be caused by the irradiation (and subsequent formation of color centers) of salt (NaCl) by the intense bombardment of the Jovian magnetosphere1.
Image Credit: NASA/JPL/University of Arizona
For a deeper dive into the cause and diagnostic potential of color, I recommend Earth Materials: An Introduction to Mineralogy and Petrology by Klein & Philpotts