Many butterflies exhibit structurally coloured wing patches that are stunningly bright and iridescent in their appearance, yet functionally obscure. These colours are often exaggerated in males, which suggests a sexually selected origin. We studied the visual properties, morphological basis, and interindividual variation of structural wing colouration in the common eggfly, Hypolimnas bolina L. (Nymphalidae). Males of this territorial species possess highly directional UV/violet colouration that fully overlaps smaller white patches on their dorsal wing surfaces. We sampled 56 males, including territorial residents and non-resident 'floaters' and assessed the properties of their structural colour using reflectance spectrometry and scanning electron microscopy. The patches reflect strongly in the UV range (300-400 nm), with a peak of ∼360 nm, and the wing scales in these regions exhibit a ridge-lamellar surface architecture that has known function in other species as a multiple thin-film interference mirror. Peak UV brightness was variable, and both brightness and peak hue varied systematically across age classes. UV brightness was also related to hue independently of the age-related variation. Territorial residents possessed duller UV markings than their non-resident contemporaries, which is not consistent with exaggeration due to male-male competition. The high phenotypic variance is, however, consistent with a putative role for this male-limited trait as a sexual ornament.