Once all the preparation was done, it was time to get on with the painting proper. First of all, I drybrushed the chainmail with Citadel Mithril Silver, then touched up the white primer where my drybrushing had strayed - as it inevitably does.
Then it was time to paint the flesh - Foundry Flesh 5B, with the severed head looking suitably pale in 5C. A blob of 5A inside the severed head's mouth, and two spots of white for the upper incisors, as well as a bit of gore where the head had joined its body.
Next came clothing. Cassius Dio has this to say about her appearance:
In stature she was very tall, in appearance most terrifying, in the glance of her eye most fierce, and her voice was harsh; a great mass of the tawniest hair fell to her hips; around her neck was a large golden necklace; and she wore a tunic of divers colours over which a thick mantle was fastened with a brooch. This was her invariable attire.
The 'tunic of divers colours' had a light green base, a bold check of a darker green, lined with a dark blue-grey, the whole being set off by either a yellow or red pinstripe going through the middle of the light green squares. The cloak was to be a dusty blue, with stripes of dark blue-grey and bone. Her bodyguard was to have yellow trousers with blue stripes and also a red pinstripe. Here's a scan of some tests I did (together with the base colour for Boudica's hair) on a scrap of paper to see which colours I liked best:
I decided, after consulting my wife, son, and daughter (and getting three opinions, all different from each other and different from my own), to go with B and 2 and to use the red pinstripe, as the yellow didn't really show up. So here is the first stage: