Typically, inorganic pigments are crystals of metal compounds (often oxides). Transition metals have their bonding electrons in d orbitals – when the metal is bonded to groups of atoms, this can cause the d orbitals to ‘split’ into two levels of energy. The electrons in these orbitals can then absorb energy to move between the two levels – the energy gap between the levels determines the wavelength of light absorbed. As not all of the light hitting the compound is reflected, this means we perceive them to be coloured.