Color constancy is a Gestalt principle of perception that suggests that the context in which an object we are viewing appears in, influences the way we perceive the color of that object. (Wow, is that as wordy and vague as I think??) Here is an example. You are looking at a bowl of fruit, which has in it a bright red apple, bananas, grapes, and some mangos. If you saw only a small portion of the apple, but did not know that it was an apple, the color would appear to change a little as the light changed. However, once you know it is an apple, you will still perceive the color as bright red even when the light changes a little (really).