Carotenoid-based plumage coloration of birds has been hypothesized to honestly reflect individual quality, either because carotenoids are difficult to acquire via food or because of a trade-off in allocation of carotenoids between maintenance and signaling functions. We tested whether differential foraging ability is a necessary precondition for maintaining individual differences in carotenoid-based plumage coloration in male greenfinches (Carduelis chloris). Wild-caught birds were brought into captivity, where half of them were supplemented with carotenoids while the other half was maintained on a carotenoid-poor diet. Color of the yellow parts of tail feathers, grown under natural conditions, was compared with that of the replacement feathers, grown in captivity. Carotenoid supplementation increased feather chroma (saturation). Color of wild-grown feathers significantly correlated with the color of lab-grown feathers. This result demonstrates the existence of a significant component of variation in carotenoid coloration, which reflects physiological qualities or genetic differences among individuals independent of foraging ability. Among both experimental groups, plasma carotenoid concentration during feather growth strongly correlated with chroma of the feathers grown in captivity. This indicates that carotenoid-based plumage coloration can reveal circulating carotenoid levels over a very wide range of concentrations, suggesting the ample signaling potential of such a mechanism.