I noticed that, because on the +farm map (f_road_h1.map, line 348) "skin" "1" is defined for tractor.md2, which should be the red (tractor1.png) skin, but still in-game the green tractor is rendered, which would be skin 0 (tractor.png).
I checked the skinpaths of tractor.md2 - there everything is correct, so IMHO the bug has to be hidden somewhere in the code...