Poor Rick.
Dave Ward is stop on.....Panzer Grey not Field Grey.
Anyway, whats the breeze re- getting the right shade? Even the paint would be slightly different shades in the tin, depending on the manufacturer.
Add such things as the weather, sun bleaching, mud splashes, dust, wear and tear, touching up the old paintwork, how many coats has been applied and how thick etc, etc.,it doesn't matter, as long as the shade is something like. The vehicle itself will have different shades depending on the surfaces exposure to the elements and abrasive reaction - and big boots!
I could go on but I think you get the picture......
Just get it something like. There again I'm just a near enough merchant!