I really like the painting. I would not had chosen complete white hair, but that's matter of personal taste. I also would say that it is quite a fantasy character as most details are not based on any sort of evidence (f.e. the scale armour etc) and I really don't know why any "barbarian" must have overweight and a long beard.
Concerning the colours: It's quite possible to achieve brilliant, deep and vidid colours with natural dyes. Artificial dying was invented in the very end of the 19th century and nobody denies bright colours in the Napoleonic era. Of course some chemical processes were better understood and the dyes were coming (nearly) from around the globe, but colors were deep and vivid.
In the early pre-mediveal ages the dying was more tricky, but is was possible. Take a look at these dyed wool, which was was dyed with iron age dyes and techniques ( Lejre Museum Denmark).