Why Can't We See Colors in the Dark?

Light from the sun or from any very hot source is called white light. But, as Newton was the first to show, white light is really a mixture of light of all colors. When a beam of light is made to pass through a glass prism, we see all the colors of the rainbow   red, orange, yellow, green, blue, and violet. Each shade blends gradually into the next without a break. This spread of colors is called a spectrum. 

These colors are present in sunlight to begin with, but show up only after being spread out by refraction in the prism. Each color is refracted a slightly different amount, red least and violet most. This spreading out is called dispersion. Without dispersion, the mixture gives the appearance of white to the eyes. Color is determined by the wavelength of the light(like the distance between one crest and the next in a wave travelling on water). The shortest visible light waves are violet: the longest are red.

Most of the colors we see in our surroundings are not of a single wavelength, but are mixtures of many wavelengths. When white light falls on a object, some wavelengths are refracted, and the rest are absorbed by the material. A piece of red cloth, for example, absorbs almost all wavelengths except a certain range of red ones. These are the only ones that are reflected to your eyes, so you see the cloth as red. So color is a quality of light, it does not exist apart from light. All our color sensations are caused by light rays entering our eyes. All objects are seen by reflected light, and the colors that they show exist in the light and not in the object.