Green plumage is nearly always an optical trick. A layer of feather structure scatters blue light while an underlying yellow carotenoid absorbs the rest, and the eye reads the combination as green. Crush such a feather and the green disappears. Turacos are the exception: turacoverdin is a genuine green pigment, and the related turacin is a red one, both built around a porphyrin ring holding copper.
The pigments were identified in the nineteenth century, when chemists found that red turaco wing feathers gave up colour to alkaline water. This produced the enduring claim that turaco colour washes out in the rain. Under natural conditions it does not; the effect requires alkaline solution rather than plain water.
The great blue turaco carries turacoverdin only in limited amounts and is largely blue-grey, so the family's signature pigments are best seen in the smaller green species and their crimson wing patches.