[...] I read a trilogy based on Boadicea by Manda Scott who has her as "copper haired". Other than that I guess the color scheme is pretty much open to interpretatation which is a help.
I realise this figure is a romanticised portrait of her (I doubt she'd have looked quite as babelicious as this for real) but the following might be of interest. There is only one ancient description of her (in a book on the history of Rome written in Greek) and unfortunately for us the word used (ξανθός , ή, όν) to describe her hair means "yellow, of various shades, frequently with a tinge of red; brown or auburn". It was also used to describe a field of grain, a flushed complexion and as a word for dyed hair. [Liddell & Scott's "A Greek-English Lexicon", Oxford, 1940]
If it was me, I'd make her a dark blonde but with obvious bright red dye on her hair (ginger ends with blonde roots, for example, or red streaks). Other "barbarian" leaders were known to do this when they went to war. Julius Civilis, who led the Batavian revolt against the Romans at around the same time, famously did the same.
If anyone is interested, the description of her is by Cassius Dio (in Roman History LXII) who describes her thus:
"A British woman, Buduica*, of the royal family and possessed of greater judgment than often belongs to women. [...] In person she was very tall, with a most sturdy figure and a piercing glance; her voice was harsh; a great mass of [blonde/auburn/dyed] hair fell below her waist and a large golden necklace clasped her throat; wound about her was a tunic of every conceivable color and over it a thick cloak had been fastened with a brooch. This was her constant attire. She [...] grasped a spear to aid her in terrifying all beholders" (H.B. Foster's 1906 translation)
* She's also called Bunduica, Boudica and Boudicca in other manuscripts.
Since Dio actually wrote well over a century after "Buduica" there are some grounds for supposing that this is less a true picture of the lady herself and more of a stereotyped Roman description of "barbarian" Celtic women. But then that's pretty much what the figure is too.