This could be due to missing or defective cones that respond to green light. The individual sees colors as if only the red and blue cones are functioning. When an expert magician performs disappearing coin tricks or when a stick appears to bend as it enters the water, we are reminded of how easily we can be tricked by what we see. The viewer is quite literally mocked into seeing something that is different from what is actually there. There are many kinds of optical illusions.
Some examples are illusions of size, shape, direction, motion, and perspective.
Probably one of the most common geometric optical illusions is the one at the right. Even when the viewer is familiar with the above illusion, it is easy to be fooled by the apparent length of each segment. Below is an illusion of depth created with the Mueller-Lyer model. Depending on how the viewer sees perspective, the vertical line of the room may appear farther away than the vertical line of the building. Although the vertical lines produce equal retinal images, one seems closer to the viewer than the other does.