To understand whether early phenotypes are adaptive, knowledge of the environmental factors involved in their variation and the derived benefits from their expression is needed. Temperature and sunlight are considered two major selective forces influencing phenotypic coloration of birds at a global scale. However, within-population color adaptations in response to sunlight-temperature variation have been scarcely investigated and their acclimatization capacity is currently unknown. In addition, the sexes differ in their sensitivity to environmental factors during growth. This study evaluates how melanin plumage coloration varies in relation to sex and sunlight exposure in developing griffon vultures Gyps fulvus, and how this correlates with survival. The results show that individuals growing in nests more exposed to sunlight developed paler plumages as predicted by the thermal melanism hypothesis. In addition, paler males, but not females, have lower survival probability. Finally, a significant sexual dichromatism in fledgling plumage was observed, with females being paler than males. These results reveal within-population color variation related to sunlight-temperature conditions, probably as a capacity for local acclimatization through color plasticity and also provide evidence of sex differences in fitness optima for this trait under ecological pressures.