Photochromes are dyes that change their colour depending on the light they receive. When light is switched off they can either remain in their photoinduced state (P-type photochromes) or turn back to their original state (T-type photochromes). T-type photochromes may colour when irradiated, bleaching when light is moved away (direct photochromism) or discolour under irradiation, recovering their colour in the dark (reverse photochromism).