I did some detective work on John Deere green: Valspar and Rustoleum both offer one, they are both the post-1989 lighter shade. Rustoleum has it listed under their "farm and implement" specialty paint. Valspar also sells a hardener which can be used with any alkyd oil-base paint including theirs.
I also want to try mixing the Rustoleum garden variety "Hunter green" with some of their "Sunburst yellow" and see how close to JD green I can get.
Mark S.
ps I saw several nice machine colors in the "Farm and Implement" series: Kubota orange, and some good reds too.