Feather Colouration 
Colours can be caused by pigments, light refraction, or both.
Pigments: 

Carotenoids
o    Responsible for reds, yellows, and oranges
o    Produced by plants, acquired by eating plants or something that has eaten plants
o    Interact with melanin to form olive-greens
Melanin
o    Tiny granules in both feathers and skin
o    Range from dark black to reddish brown to pale yellow
o    Strengthens feathers
Porphyrins
o    Produced by modifying amino acids
o    Fluoresce bright red under UV light
o    Produce pink, browns, reds, and greens
o    Each porphyrin has a similar but unique structure
 

 

Structural colours:
 Iridescent feathers
o    Refraction of incident light in feather barbules
o    Change in viewing angle causes change in apparent colour 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Non-iridescent feathers
o    Tiny air pockets in barbs of feathers cause incoming light to scatter causing non-iridescent colour
o    Almost all blues are caused by this.
 

 

 

 

 

 

 

 

 

 

 

 

Ultraviolet feathers
o    Some feather colours are only seen in the UV range
o    Birds can see further into the UV range than humans

 

 

 

 

 

 

 

 

 

 

 

 

Source: https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/