How does the Purkinje shift change colour perception at low light levels?

The Purkinje shift describes the change in the eye's spectral sensitivity as light levels transition from photopic (daylight) to scotopic (night) conditions.1

In daylight, vision is mediated by cones, which are most sensitive to wavelengths around 5550 angstroms (yellow-green). As light levels drop into the mesopic range—where both rods and cones are active—and eventually into the scotopic range, the eye's peak sensitivity shifts toward 5000 angstroms (blue-green), the absorption maximum of the rod pigment rhodopsin.1

This shift fundamentally alters colour perception:
* Luminosity changes: As darkness increases, the rod luminosity scale begins to prevail over that of the cones.1
* Perceptual shift: Because rods are more sensitive to shorter wavelengths, blues appear relatively brighter in dim light, while reds—which are less stimulating to rods—appear significantly darker or black.1

Because rods do not provide colour vision, this transition results in a loss of chromatic detail as the eye relies increasingly on the monochromatic signals of the rods.2