The effect occurs because the color-sensitive cones in the retina are most sensitive to green light, whereas the rods, which are more light-sensitive (and thus more important in low light) but which do not distinguish colors, respond best to green-blue light [REF].
The Purkinje effect occurs at the transition between primary use of the photopic (cone-based) and scotopic (rod-based) systems, that is, in the mesopic state: as intensity dims, the rods take over, and before color disappears completely, it shifts towards the rods' top sensitivity [REF].