New balls don't appear under the one you're moving there is a buffer zone around each ball the keep other balls from forming too close. This makes it more likely for new balls to show up in wide open areas. You can't move too quickly without risk even if there isn't anything there. Also you can reposition your window to drag larger balls off screen if there isn't enough room on a side to pull it off. There are five colors total, I've never gotten all of them but I've seen scores indicating someone has. (Or they cheated.)
 "Ferris of them all"){kind=icon}
 "Magnetic Putt"){kind=icon}
 "The Merry Dance"){kind=icon}
 "Glom"){kind=icon}
 "HangGlider Dude 1"){kind=icon}
 "PiXeL 2"){kind=icon}