1. Condition-dependent expression of ornamental traits is a fundamental assumption of theories on the honesty of sexual signals, and it is widely assumed that condition-dependence is a signature feature of ornaments. 2. Some of the best understood condition-dependent traits are the striking carotenoid-based plumage signals of male birds, yet little is known about the many less conspicuous, less elaborate carotenoid-based plumage colours that often comprise large parts of the plumage. 3. We examined colour (reflectance) of carotenoid-based plumage in male greenfinches that were provided with naturalistic diets with relatively low and with enhanced lutein availability during their annual moult. Using a variety of objective colorimetrics, including physiological models of avian colour vision, we compared experimental effects and general condition-dependence on the contrasting bright yellow tail patch, the yellow-green breast as well as three duller, yellow- to olive-green patches (back, crown, rump). 4. Irrespective of the analysis method used, we found consistent and large diet effects on the reflectance of the tail, much weaker effects on the reflectance of the breast, and no significant effects on the other three plumage parts. Likewise, we found that only the colour of the tail was strongly associated with circulating (plasma) lutein concentration, as well as with general condition (body mass, haematocrit). 5. Our results suggest that, in accord with current theories on the signal honesty, the striking yellow tail patch of the male greenfinch appears to be particularly well-adapted to signal information on carotenoid availability and general condition of the male during moult.