Snobrd,
Xtra Color markets a color they call BAC707 Boeing Grey, number X301.
I've used it on the wings and horiz stabilizers of a 1/144 scale 737-200, and I think the color is seems just a
little dark - well, it's a little darker than the grey on the upper surfaces of Qantas 737's, but it looks pretty good - closer than any other paint I could find. I've no idea what the FS number would be.
The XtraColor paint must be thinned with white spirit (a refined form of turpentine) and they sit down very nicely. It's the first time I've used it and highly recommend it.
Cheers,
LeeTree