I had a play too. It was fun.
The tractor is mostly red and magenta with a bit of blue. The rest of the image is green & cyan + few isolated patches of blue + red. So you're better off working on the colours separately. You don't get the halo/boundary artefact between the tractor and the surrounds that way. Only a little bit of manual (de)selection is needed for the few elements with blue & red in the background.
I've attached my GIMP tweaks with background grey, tractor grey and another with the tractor dipped in hydrochloric acid.
