May 31st, 2007
Colour is caused by selective absorption of certain wavelengths and reflection of the rest. The most familiar example would be that of phosphorus, which has three allotropes each of which have a different colour; white, red, and black, and several variations within these three basic forms. The structure of the red form is not certain, although one form of red phosphorus has been elucidated there is no clear evidence that the other red forms are the same.
All of the allotropes have molecular structures with corresponding molecular orbitals. It is the electrons within these orbitals that absorb light that promote electrons to higher molecular orbitals giving the characteristic colour by subtraction from white light.