I manage a paintball site and an airsoft site. The paint is the only part of the day we make any money on for the paintball site. Your green fee pays the rent/insurance/gas costs, if we're lucky. That's why paintball games are usually designed for you to use as much paint as possible.
Sites don't allow paint to be brought in so that they can earn something. Also, the paint you have bought may not be suitable for the temperature/weather or may jusyt be old and crappy. Wrong type of paintball or too old can cause jamming/chopping of ball in marker or they simply won't burst on impact.
As mentioned by others, call the site, find out if they will do you a deal in advance on bulk paint.
Airsoft is becoming far more popular now as an alternative, in large part due to cost. (Certainly at our sites anyway) Our rental price includes all ammunition for the day, at £40. (also get kit, lunch etc…) Even without that, 3000 airsoft bbs cost between £7 and £10 retail, so hardly bank breaking.