Polarized optical absorption spectra of single crystals of the two dimorphic Mn₂AsO₄(OH)-minerals eveite and sarkinite show that minor concentrations of Mn³⁺ at the octahedral site determine the colour and cause the distinct pleochroism in green and yellow of eveite. In the sarkinite spectra only absorption bands due to spin-forbidden d-d transitions in Mn²⁺ at six- and five-coordinated sites are observed, which produce a weak flesh-red mineral colour and a very faint pleochroism.