The proper colour is "National School Bus Chrome Yellow, Federal Standard 595a number 13432"
I used Humbrol Yellow #69 with a few drops of Humbrol Scarlet #60 added to the can. Basically I added red until it matched the colour on a small diecast school bus model I have. I prefer to use hobby paints, but I'm sure an auto paint supplier could mix up a spray can for you.
Oh yeah, my bus...