The application of modern spectrometry to the study of avian colour variability has revealed ignored patterns of colour variation such as male‐biased sexual dichromatism and seasonal variability in the plumage. However, the variation in the achromatic property of such traits, that is in the total light reflectance of the spectrum (i.e., brightness), has commonly been overlooked. The evolution of signals based on brightness should be favoured in those species that are active when light is scarce, i.e. at dawn and dusk. The eagle owlBubo bubois monogamous and apparently monomorphic in plumage‐coloration. In this species, sexual and territorial call behaviour is mainly performed at dawn and dusk, during which a white patch on the throat is repeatedly exposed at each call. We measured the total light reflectance of the feathers of this badge in 39 eagle owl specimens from museum collections. We found seasonal variability and sexual dichromatism in the brightness of the plumage badge. The total reflectance of this trait peaked during the territorial‐mating period. Moreover, females showed higher values of brightness than males, in agreement with the reversed body size dimorphism present in this and many other raptor species. Finally, female but not male body size was positively correlated with white badge reflectance.