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• In the Azores, six phenotypes of the butterfly Colias croceus (Pieridae) exist. • Pterin pigment as well as thin film optics determine the wing colouration. • Three forms are characterized by the absence of the red pigment erythropterin. • A ' cremonae ' polymorphism probably coexists with the well-studied 'alba' form. • The phenotypes' characteristic genes interfere in the pterin biosynthesis pathway. The pierid butterfly Colias croceus (Geoffroy in Fourcroy, 1785), established in the Azores archipelago, is polymorphic with six forms, C. croceus f. croceus ♂ and ♀, C. c. f. cremonae ♂ and ♀, C. c. f. helice ♀, and C. c. f. cremonaehelice ♀. We investigated the optical mechanisms underlying the wing colouration of the butterflies by performing spectrophotometry and imaging scatterometry of the variously coloured wing areas and scales. The scale colouration is primarily due to wavelength-selective absorption of incident light by pterins expressed in granular beads in the wing scales, but thin film reflections of the scales' lower lamina and scale stacking also contribute. Three forms (croceus ♂ and ♀ and helice ♀) are consistent with the patterns of the well-known ' alba ' polymorphism. We postulate the coexistence of a second polymorphism, ' cremonae ', to understand the three other forms (cremonae ♂ and ♀, and cremonaehelice ♀), which are characterized by the absence of red pigment, presumably due to the differential blocking of erythropterin expression. [ABSTRACT FROM AUTHOR]