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Why do rod cells not provide colour vision?

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details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

Rod cells do not provide colour vision because they form a homogeneous population that contains only one type of photopigment, known as rhodopsin.12

Because all rod cells contain the same photopigment, they cannot discriminate between different wavelengths of light. While rods are highly sensitive to light and function as wavelength discriminators to a limited extent, they cannot uniquely identify a specific colour. For example, the brain receives the same signal from a rod cell regardless of whether it is stimulated by a specific intensity of blue light or a different intensity of red light.1

In contrast, colour vision is made possible by cone cells, which contain three different types of opsins that allow them to respond to distinct blue, green, and red wavelengths. The brain interprets colour by comparing the ratio of excitation across these different types of cone cells. Because rods lack this diversity in photopigments, they are limited to perceiving brightness and motion rather than hue.312