On the blue coloration of vertebrates.

Abstract in brief

• The blue coloration in vertebrates is primarily a structural color resulting from the scattering of blue light, either within cells or from surface structures like collagen arrays or feather formations.
• In lower vertebrates like fish and amphibians, iridophores with reflecting platelets and dermal collagen are key to producing blue hues, while in birds, feather structures predominantly contribute to blue scattering.
• Interestingly, there is only one known case where blue coloration is caused by a blue pigment within a cell, and for amphibians, reptiles, and birds, green colors are produced by a combination of blue light scattering and yellow pigmentation.]