Please don't buy the standard tractor paint made by Valspar. It is the worst oil based enamel I've ever used. I painted my first tractor with it and within six months the sheen was gone. After a year the paint faded out to where it looked worse than before I painted it. After two years the paint failed completely and had to be repainted. I would recommend that you go to Sherwin Williams and get the paint. They won't have the color codes. You will probably have to make a color sample with some rattle can paint.
Having said that to use an oil based alkyd enamel you should thin it as little as you can and still spray it. The more thinner you use makes the paint dry slower and ruins the integrity of the paint. When spraying it most paints recommend naphtha. With the Valspar hardener it is recommended to use two ounces per quart. The hardener is very helpful. It makes the paint perform like the old lead based paint. When you paint using a hardener use it in a well ventilated place and especially use a chemical respirator. The hardener contains cyanides.