There are actually only two different color chickens in this coop. See the proof below.
There may seem to be four different color chickens: cyan, green, magenta and red-orange. In fact, there are only two. The cyan and green are the same color, and the magenta and red-orange are the same color. Here’s proof:
This is known as a chromatic dungeon illusion. It takes advantage of both color assimilation and color contrast. Taking the “cyan” chicken as an example, the strongest effect is of the magenta bars assimilating with the green chicken, making it appear more blue. However, the orange background also contrasts with the green chicken, adding to the blue effect. With the “red-orange” chicken, the yellow bars assimilate with, and the blue background contrasts with, the magenta color, making the chicken appear red-orange.
Chromatic illusions are explored in depth in the book Trick Eyes, by Akiyoshi Kitaoka. The book contains some of the best work by Kitaoka, a psychology professor who has created some of the most powerful illusions of color and motion. It is also a how-to guide that explains in a friendly tone how you can create some of the simplest illusions on your own.