A colour illusion takes place when the colour we see gives information which, if tested otherwise than by vision, would be found to be misleading or confusing or even meaningless. An illusion is also set to occur when colour changes for no reason immediately apparent to the observer. A colour illusion causes us, for the moment at least, to be deceived by what we see; but the extent to which we are deceived depends on the extent of our knowledge and experience. What is an illusion for one person may not be for another.
Colour is a sensation in the brain and exists only when there's someone present to experience the sensation; and since it is usually caused by the reflection of light waves from surfaces onto the eye it changes with every change of light or surface. Colours become bluer, yellower, darker or lighter according to whether, for example, they are seen in open daylight, in artificial light, in twilight or in sunlight, and they are hardly ever exactly the same for any length of time.
The brain achieves an extraordinary feat when it gives a consistant and uniform appearance to the things we see in so many different lights and contexts. The brain's faculty for giving continuous order to the mass of unassimilated data that it is presented with by the senses is called constancy.
The brain's attempt to recognize beneath the changing conditions of the outside world, certain surface qualities that are relatively constant can be described as colour constancy. It's success depends upon stored experience and the evidence not only of sight but also of our other senses.
Illusion is, physiologically speaking, a glimpse behind the scenes at unassimilated sense data which has not, as it where, been put into proper perspective by constancy.