The optic nerve and the optic chiasm transmit the sensory information for vision from the eye to the occipital lobes of the brain. Since these structures transmit the signals to the brain their function is not affected by the levels of light.
The cone cells are photoreceptors, they respond to different wavelengths of light (depending on the type of cone cell, they detect a different wavelength) and are responsible for color vision. They work best in relatively bright light.
The rod cells are also photoreceptors of the retina and function better at lower intensities of light and are almost entirely responsible for the ability to see in the dark, i.e. under low light conditions.
Considering the function of each structure the complete sentence will be:
It is harder to correctly identify colors in the dark because Rods primarily function in the dark, and they sense shades of black, white, and gray.