|The fossil, Bohaiornithidae, is exceptionally well preserved|
The prehistoric bird, which was found recently in China, may have puffed up its feathers like a peacock.
The bird's feathers are "remarkably preserved", including the chemical that gave them sparkle.
The animal belongs to a group of early birds known as enantiornithines, which lived during the Age of the Dinosaurs.
All known specimens come from rocks in Liaoning, China, which have yielded numerous fossils of feathered dinosaurs, primitive birds and pterosaurs.
"Many enantiornithine birds possessed ornate feathers," said lead researcher Dr Jennifer Peteya of the University of Akron, Ohio, US.
"This new specimen shows that some enantiornithines also had iridescent feathers and unlike most modern birds, these flashy ornaments developed before the animal was fully grown."
Scientists have limited knowledge of the plumage of birds from the time of the dinosaurs.
Melanosomes can be seen in the fossil - microscopic structures that give rise to a broad range of colours in modern birds.
They also produce iridescent effects, according to the way they reflect light.
This is the first time evidence for iridescence has been found in enantiornithines, said Dr Stig Walsh, senior curator of vertebrate palaeobiology at National Museums Scotland.
The colouration is used by birds today mostly for sexual selection.
"Although this particular individual was a very young adult, its tail feathers were already long and formed a kind of streamer," said Dr Walsh, who is not connected with the research.
"Again, this kind of extravagant feather array, like the tail feathers of peacocks, is usually used for mate attraction.
"It seems this bird was an adolescent out on its first attempt to 'pull', so to speak."
The research is published in the journal Palaeontology.