The mouth coloration of passerine nestlings is hypothesized to attract parental care by increasing the visual conspicuousness of begging chicks and/or by signaling the reproductive value of nestlings. Specifically, carotenoids are often hypothesized to mediate the latter relationship. In House Sparrow (Passer domesticus) nestlings, we confirmed both the presence of carotenoids in rictal flanges and a positive relationship between carotenoid concentration and the intensity of yellow coloration. This carotenoid-based coloration was positively associated with nestling mass and with plasma carotenoid concentration. Red gape coloration also revealed titers of circulating carotenoids. Carotenoids reduced the overall brightness and the UV reflectance of flanges, an effect that may limit the detectability of carotenoid-rich mouth colors and the ability of brightness and UV coloration to function in communication. For example, flange brightness, likely the primary determinant of conspicuousness, was positively related to nestling mass and levels of circulating carotenoids but only when the reflectance effect of carotenoids was removed statistically. We found no evidence that UV coloration positively reflected nestling condition. Most aspects of mouth coloration were influenced by Julian date and differed among broods, suggesting that colors can capture information about temporal and nontemporal features of the environment experienced by nestlings and, furthermore, could have a genetic component.