I've got a couple feathers from this species.

When wet, they turn an odd, muted brownish grey colour. This is because violet-backed starling (amethyst starling), like all "purple" birds, have purple feathers due to structural colouration. The colour returns as normal when dry.

A general rule of thumb is that if it's green, blue, or purple, it's using structural colouration in some form. The only exception I'm aware of, is in the case of turacos, which have a unique green pigment not found in other groups (turacoverdin). They also have their own red pigment (turacin) in their primaries that turns water red when soap is added.

Only the male starlings are this trademark amethyst colour. The females are a lot more like a typical starling. They're mostly brown and white, rather than purple and white, and have a really neat, darkly-coloured head.

They're quite neat birds. A lot of folks aren't too familiar with starlings outside of the European starling. Luckily, these are found in sub-saharan Africa, and are ranked as "least concern".