They have not been categorised scientifically to date. Carotenoid pigments are responsible for colours ranging from yellow, through orange to red and including pink. The melanin within the medulla is commonly described as 'background melanin'. The medulla of the barb is important as, in many species, it forms a special structure known as the cloudy layer in its outer region. The cloudy layer is responsible for the light distortion effect known as Constructive Interference.
Constructive interference is what makes feathers appear blue or green to the human eye.
This light distortion effect had always been attributed to Tyndall effect or more correctly Raleigh Scattering. But recent work by Richard Prum et al has demonstrated that the physical mechanism should be classified as interference rather than scattering. For the cloudy layer to produce its effect, it requires a background of melanin behind it. This melanin is naturally black, but because of its depth within the feather it will appear grey in colour. Except when the cloudy layer is present, in which case if will appear as various shades of blue through to violet.