SilverMK2 wrote:Vallejo have comparison colour charts for
GW's paints (I think new and old), P3, some Reaper paints and possibly Tamiya for each of their ranges (off the top of my head).
I think that might be your best bet.
Yep, they seem to have quite comprehensive chart available here - many Vallejo colors seem to be matched to U.S. Federal Standard colors, but not all of them:
http://www.vallejo-farben.de/download/CC073_rev03.pdf
Also, most art stores have painted color charts available, but each manufacturer has their own, and I don't know if they can be matched to USFS or RAL.