Raven-feathered Archaeopteryx

For years, palaeontologists have speculated about Archaeopteryx's colour scheme, without knowing for sure. Artists have painted it in every shade from tan and rusty brown, to bright greens and blues.

All this speculation could soon end, with new ways to detect prehistoric colours. 

...The feather [shown above] was most probably black. While the full colour pattern of Archaeopteryx has yet to be uncovered, Carney noted that melanosomes on the black feather have structural properties which may have strengthened the feathers for the demands of flight. The miniscule structures which hide the secrets of prehistoric colour were not just for show.

From New Scientist. Other prehistoric birds and feathered non-avian dinosaurs were more colorful, and some later fossil feathers display iridescence.