In 28mm, you tend to see certain tropes used. So Romano-British will look late Roman. "Celtic" (Welsh, Irish/Scotti, Picts) will often have bare legs or trews, rather than trousers. No shoes. Rolled cloaks, painted in simple plaid type schemes. The Welsh will often get short hair and moustaches but no beards. The Picts lots of tattoos, perhaps naked (Attecotti). Crossbows and square or I shaped shields. The early Germanics can get small buckler shields. Vendel type helms. Clothed with pelts and furs.
These are just ways to create artificial differentiation. They often have little in the way of substantive evidence. If there were clear differences in fashion they would have been at a tribal level, not at a more macro "cultural" level.
So my take is basically take the tropes that you like and dump those you don't. I tend to use some form of unifying colour scheme for a specific warband. So clothing, shield design/shape, banners etc. It's all rather arbitrary though. I happily use some tropes but totally reject others. My early Saxons ain't getting those silly little bucklers.