Our eyes give us a very subjective view of the world. This subjectivity leaves us susceptible to a variety of optical illusions. The fact that we are not very good at estimating the absolute brightness of a region, but have a strong ability to detect differences between neighboring regions causes simultaneous brightness contrast illusions. This type of illusion makes the perceived brightness of any region in a complex image vary depending on the brightness of the region’s immediate surroundings.
The illustration below shows how our perception of a colors in different Mediums