The original paint is available, but you have to buy a lot of it to be able to get it. I don't know the particulars but it's out there.
Some use the Case Flambeau red as it is pretty close. Some use Allis Chalmers orange but it, for the most part, is too light. I hear that there are two different colors of Allis orange but I've not seen it myself. Some use Duplicolor Chevy engine orange 1620. It looks good but it too is only close and probably a bit too bright. The reason you can't find an exact match anymore is because the paint they used had lead in it so it's hard to just go buy it anywhere. Can't get an exact match without the lead. The Case paint is probably your best choice.