Answer:
Well it depends on how big your backyard is and where you live. If you live in a neighborhood chances are your backyard is wayyyyyy to small, but as long as the paintballs remain in your yard and no property is damaged than it is technically legal. Now if you have a lot of property than have fun. Note if you live in a rich neighborhood like a country club or any gated community, old people will probably poop on the party and call the cops, but the cops cant do anything except tell you, you cant shoot anymore.