Why do all colors appear grey in dark for your eye?

In complete or near complete darkness, when the amount of light reaching the retina is very low or absent, the cone cells in the retina become inactive. Cone cells are responsible for color perception and work best in bright or well-lit conditions. They are primarily sensitive to different colors of light.

On the other hand, rod cells in the retina are more sensitive to low light levels and work better in dim or dark environments. They are not capable of perceiving colors and therefore, everything appears grayscale. In extremely low light conditions, only the rod cells are functioning, and color perception is lost, resulting in a lack of color discrimination and a grey or colorless appearance of the surroundings.