OK, these took a bit longer than anticipated (it took another convention to get my painting kickstarted again - this one the Red Barons’ convention), but here they are in finished form:
The quickshade method works a charm - you just have to remember to use lighter colours than normal. In three shade terms, I mostly use the highlight colour only on these figures. For Steven (the commenter below), these are the colours I’ve used on these figures before dunking them in Quickshade (the medium strength version):
As I was worried about the historical accuracy of pitting these Romans against my Celts, I’ve bought a set of Caesarian Romans from Wargames Factory, which are correct historically. The idea is that I’ll use these Imperial Romans and the Caesarian ones where appropriate, supplemented by the same set of auxilia and cavalry.