During the day, or
bright light, the muscles in the iris shrink the size of the pupil (a hole) to
keep the eye from being overwhelmed from too many light rays. The eye cones primarily respond to colors, and
during the day, in bright light, colors appear vibrant.

During the night, or dim
illumination, the muscles of the iris open the pupil (a hole) to let in more
light. With less light the rods take over, they work well in poor
light (until you hit 40, then you need reading glasses for menus in dark
restaurants!) but the rods don’t detect colors like cones, so this explains why
you don’t see colors as well in the dark.