I don't consider this a spoiler, because I spent all of Shades of Grey trying to find geographical clues that would help me place the action. This ended up being quite a distraction, so I consider this discussion a favour to readers, to save them time and cognitive load. However, if you'd rather not know anything going in, please don't open the cut ...
In Red Side Story, we learn East Carmine was originally Rhayader ...
I presume, therefore, that High Saffron was once Aberystwyth, given that it's the nearest significant settlement on the sea, and they travel over a high scrubland to get there ...
... possibly even taking the Perpetulite road laid on the bed of the B4574. Or possibly they take the A470 referenced in the chapter "Perpetwlait & Vermeer" (p.319, UK edition) – this is the main road that runs N/S through Rhayader, and does go to Aberystwyth, but less so over the high ground described in the story.
Taking that Purple Regis was once Lyme Regis, we know the Southwest at least is Green Sector, but whether the Green Sector covers the whole of Southern England from east to west, or whether it's just the West Country and the Southeast is Yellow Sector (because, come on, it would be) depends on whether Emerald City is in fact London and whether it has to be in a sector that aligns with its colour. (Neither Purple Regis nor High Saffron do, so we know it's not compulsory.)
So, where is Vermillion? It seems to be the nearest major town to East Carmine, so is it Shrewsbury? Hereford? Abergavenny? I will have to read Shades of Grey again to pick up any references that might be helpful, but maybe you have an idea?
FWIW, present-day, non-Chromatic Rhayader looks really nice! I would definitely add it to my list of extremely niche places to visit.
