The adult peacock has about 200 tail feathers, with 170 of those having the precise “eye” pattern. The pupil of the eye feather is a dark purple and the iris is blue, surrounded by bronze and green colors.
How is this pattern made? These patterns are not pigmented, but are structural colors made by an optical effect called thin-film interference. The strands (barbs) of the feather are transparent; however, the light bounces off and, depending on the thickness of the strand, different colors reflect back and produce the colored pattern we see.
A single barb can vary from many colors, yet all the barbs coordinate with great precision to produce the eye pattern. It is amazing how God created such an intricate pattern made by such a complex mechanism!
(Source: Inspired Evidence, Stuart Burgess – The beauty of the peacock tail and the problem with the theory of sexual selection, 2001 pp.94-102)