The colors of butterfly wings are determined by the structural as well as pigmentary properties of the wing scales.Reflectance spectra of the wings of a number of pierid butterfly species, specifically the small white, Pieris rapae, show that the long-wavelength reflectance of the scales in situ, on the wing, is distinctly higher than that of single, isolated scales.An optical model explains that this is due to multiple scattering on overlapping scales by treating the layers of scales on both sides of the wing as a stack of incoherently scattering plates.The model sheds new light on the adaptive significance and evolution of butterfly wing patterns.