Polymorphisms of the juvenal plumage in birds have received little study in comparison to other plumages. We investigate a polymorphism in the juvenal plumage of the Yellow Warbler (Setophaga petechia) in southeastern Ontario, Canada. Yellow Warblers in juvenal plumage show either yellow or white plumage on their lower ventral surface of the neck, breast, and belly, and these differences have been speculated to be associated with sex. We tested the hypothesis that juvenal plumage color varies with sex by using blood samples to molecularly sex 10 yellow and 14 white nestlings. We found no relationship between plumage color and sex (yellow nestlings: 70% male, 30% female; white nestlings: 64% male, 36% female). Using the same 24 nestlings that we sexed, we measured feather color with a spectrometer and found that white and yellow morphs differ significantly in reflectance spectra of red and yellow chroma. Of 66 nestlings whose plumage color we visually inspected in the wild, 73% were white and 27% were yellow, showing that white nestlings are more common than yellow in our study population. We propose alternative explanations for the possible significance of yellow and white polymorphism in the juvenal plumage of the Yellow Warbler.