Your eye is a wonderful detector of visible light. Different frequencies of light produce different sensations in the eye which we interpret as colors.
Our eyes detect light by using light sensitive components called rods and cones. Cones are used when the light is intense, such as during the daytime. With cones, we can see different colors of light.
Rods are used when the light is very dim. Rods cannot detect colors. That is why things appear as shades of grey at night.