I've always used acrylics. Water washup and it's a polymer when it dries so it should last as long as the pumpkin should (might start cracking and peeling if outside for weeks, but I'm sure it will be fine for Halloween)
Spraypainting could be fun too. To get the solid glossy paint, I'd think that would be the best choice.
If it was a longer time off (say you started this a month ago), you could have always sealed the pumpkin with a spray sealer - they come in clear coats and have a glossy or matte finish. That would ensure the paint would be good for a LONG time.