I use 'prussian blue' for Prussians (surprise!), while for the French, I use something darker and richer, like the Royal Blue (called 'imperial' at the time, no idea why).
For historical colours (i.e. before the 10th century) I use Vallejo, some few precious Ral Partha, and paints fromoilranges, who happen to produce the same shades in acryllics.