...would it be more or less historical to paint regular british infantry miniatures in either red or blue to represent uniformed provincials?
In my opinion, it depends on the scale and detail of the figures. You could probably get away with it in 15mm or smaller.
Only general descriptions are known for most provincial uniforms, but they tend to be plainer and simpler than the regulars' uniforms. I don't know of any provincials who wore laced coats or hats. Most are shown with round cuffs and short coats without skirt turnbacks, some are shown in single-breasted coats with no lapels. Provincials were usually issued with hatchets instead of bayonets, and rarely wore leg gaiters.
In 28mm or larger scales, I'd recommend using dedicated sculpts for provincials.