Hi,
Interesting question. Firstly I've no experience of Ammo paints, out local shop stocks Gunze + Tamiya both of which I have used for wagon painting. I don't get hung up on getting exact matches as any colour pic of wagons shows huge variations.
I've used Tamiya's XF90 RAF Light grey for unfitted and XF79 Linoleum brown for fitted, they're reasonably close. Now this isn't much use really to you unless they stock Tamiya as well but might give you a starting point. A NATO brown is a passable match for late bauxite.
Sadly printed or online colour charts aren't much use for comparisons.
You could though take a wagon or paint sample in and check though the Ammo/Vallejo bottles don't exactly help.
Probably not much help but good luck.
Stu