A thing to be aware of is the two shades.
The CIE brown has variously been called "brown", "bauxite" and "red oxide".
If you look at some of the excellent colour pics online (flickr for example) by "Irishswissernie" of this world, you'll see that it was a distinct brown up to the time the ferts didn't run any more. It didn't have the distinct reddish tint you see now. This is why you'll see nowadays a reddish tint on the smear of paint behind a newly stencilled number; the underlying brown on the rest of the body is the "real" colour.
I saw a pic somewhere of a model CIE brake van in the modern reddish-bauxit-ey colour but all wagons, PW included (when not yellow!) should not carry the reddish colour that the old guard's van parked up in Limerick has. Have a look at photos online and in some colour books relating to the 1970-onwards period and you'll see there's no reddish tint.....
Incidentally, pre-brown (about 1970), absolutely everything, PW, bubbles and all, was grey of one sort or another - except tankers!