This isn't just for OP, but for lurkers too. I have a twenty-foot-long four-window shortie that I painted twice.
The first time I painted it
, picked up a few gallons of Envirotech DTM paint from an auto shop and sprayed it on, thinning it with water. I used about 1/2 gallon per coat (omitting the top of the roof, which was already white).
I literally had no idea what I was doing, and it was a pretty shitty paint job, but at least the bus wasn't yellow and I was street legal.
I drove it around for 1.5 years or so, and then painted it again
--but this time I mixed Rustoleum to get the right color (3 parts blue, 1 part green) and rolled it on. I used about 0.75 - 1.0 gallons per coat. I also added Valspar enamel hardener and mixed it on site with a paint mixer adapter for my drill. Rolling it on worked so much better for me, it was a lot easier, took less time, less tools, and I think it looks pretty good too!