Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects.
Color vision is a process by which organisms and machines are able to distinguish objects based on the different wavelengths of light reflected, transmitted, or emitted by that object. In humans light is received by the eye where two types of photoreceptors, cones and rods, send signals to the visual cortex which in turn processes those sensations into a subjective perception of color. Color constancy is a process that allows the brain to recognize a familiar object as being a consistent color regardless of the amount of light reflecting from it at a given moment.