>>14293851Because having a different color than white requires the material to absorb a wavelength that falls within the spectrum of visible light. Excluding structural effects, this means the molecules making up the substance must have an accessible excited state with a certain excitation energy. This is, firstly, not necessarily likely, and, secondly, also implies a certain reactiveness of the compound. Sometimes, a significant absorption band can be observed just outside the visible spectrum, e.g. in the ultraviolet.
