People see color before they see anything else. And trying to identify a mystery bird is no exception. Over the years I have posted photo galleries of common birds grouped by color. These photo galleries serve as guides to figuring out what bird you are seeing. And with spring migration starting, high probability of being stumped by a bird increases.
Photo Galleries: Identifying Birds by Color
Winter Feeder Birds: Identifying Blue Birds
Winter Feeder Birds: Identifying Woodpeckers
Birds by Color: Yellow Warblers
Learn the Fall and Winter Color of the Common Bird Species (Audubon website)
How the Cardinals Makes His Red Feathers