Hello Mick,
it's not quite as simple as that, grey should = unfitted and bauxite should = fitted.
But then you get to the piped that have vac. pipes but no AVB. these should have been painted in grey but some of them (IIRC) were painted in bauxite! To show the difference the pipe ends were painted in different colours. (IIRC) red for vac. fitted and white for through piped.
Then you get the XP type of van that had Vac. brakes and steam heating in the van and the type that only had the Vac. brakes and through steam heat pipes.
OzzyO.
I can't claim to have made a comprehensive survey, but the above isn't quite right. Through-piped vehicles were (under BR prior to the livery changes of 1964*), generally painted
bauxite with the through pipe, as Ozzy says, in white. Generally this applied most often to brakevans; most BR-built brakes were not actually vac' fitted but had a through pipe and vacuum valve in addition to the handbrake. In other words, the guard could operate the train brake and the van brake, but not both together! The LNER and SR had fully fitted brake vans but I am not sure that either of the other members of the 'Big Four' did.
Fitted vehicles had red painted pipes and this applied even to vac' pipes suspended below the buffer beam.
XP rating had no relation to steam heat capability; it simply signified compatibility with passenger train running. Basically, 10' or longer wheelbase and with AVB.
Passenger rated stock - a synonym for NPCCS [non-passenger carrying coaching stock] should always have had at least a through pipe for steam heat. Obviously milk tanks and CCTs didn't require steam heat!
I think Mick is slightly conflating the difference between how the wagons were operated and how they were painted. Most freight trains in the steam era lacked continuous brake, were loose-coupled and wagons were arranged in a manner most convenient for the job in hand as far as I'm aware. Brake pipes were only rarely connected up and obviously if the train engine wasn't equipped with vac' brake then it didn't matter at all what order the wagons were in. Fully or partially fitted trains - block trains of fruit or cement, for example - were different and the Rulebook made clear how many vehicles in a train needed to have the brakes fully connected up for each class of train.
Adam
* After '64 most things were meant to be Freight Brown but the braked/through pipe = brown and unfitted = grey was retained because it was useful for the operators.
Be carefully running brown and grey wagons together, my 'basic' understanding is that colour is something to do with the brakes or brakes fitted so trains tend to be all brown, all grey or split in uniform blocks.
I've been spending too much time with Tony, all this GC talk of 9F's on windcutters and the like 
