When we are in a fairly dark room, or outside at night away from lights, we can still see, but we can't see the colors of things very well. Why is that?
There are two kinds of light-sensitive organs located in the backs of our eyes: rod-shaped and cone-shaped. Both rods and cones are sensitive to light. The difference between them is that the rods allow us to see in very dim light but don't permit detection of color, while the cones let us see color but they don't work in dim light.
When it gets dark the cones lose their ability to respond to light. The rods continue to respond to available light, but since they cannot see color, so to speak, everything appears to be various shades of black and white and gray.
A curious thing is that in dim light you can see more clearly out of the side of your eye, because the light-sensitive rods are more highly concentrated off to the side in the back of your eye.
So, next time you're out on a clear night, notice how little color you can see, and how you can see objects like dim stars better out of the corner of your eye than from the center.