I ended up using Magenta and Ultramarine blue to create the violet / purple feathers which seemed to work rather well.
The Violet - Backed Starling is widely found on mainland sub - Saharan Africa. It is also known as Plum-coloured Starling or Amethyst Starling. They measure around 18 cm in length.
Violet Backed Starling measures 2.5 x 3.5 inches and is painted on 300gsm smooth Arches Watercolour paper.
Many thanks to Derek Keats for the reference photo.