The patient’s inability to distinguish between red and green suggests a problem with color vision, which is primarily associated with the functioning of cones in the retina. Cones are the photoreceptor cells responsible for color perception. Specifically, red and green color blindness is often related to dysfunction in the L (long-wavelength) and M (medium-wavelength) cones.
Therefore, the receptors that are probably not functioning properly in the patient are cones.