There are some newer studies that suggest that feather color in birds evolved over time to match the plant species that they were living amongst. This study also talks about how birds view colors and it links the visual aspect of birds and how they visually adsorb color.
I found an interesting study where they found that plumage was also associated with nest height and geographical area.
Something that could be added to the plumage section is that some studies have shown that barred pattern in feathers is more common in females than males and is mostly used for camouflage, while mottled feathers were found to be most used in signaling.