Birds' diets actually do affect the color of their feathers quite a bit. Most famously, flamingos are only that bright pink because of what they eat, and when taken out of their natural habitat, their feathers become white. Recently, there have also been reports of yellow-shafted northern flickers having feathers of a more reddish hue, and thus becoming orange-shafted, due to eating berries from a certain invasive Asian species of honeysuckle. I'm not sure if any of them use the berries to make holiday sweaters or pelt their enemies, though.