The playing field consists of: a ball, which drops under gravity and bounces; a square, which the ball is trying to reach; pre-set platforms which the ball will roll/bounce on; and custom-made platforms which you draw freehand on the screen. Simpler than some games with this feature, Paintball is unlikely to elicit gasps from its players with advanced physics, but what it does it does very well.
A quick tip—you can draw over the ball (to a certain extent). It will "jump" above the line you draw, allowing you to create slopes with greater height. You will need this when the target square is just below or at the same height as the ball's starting point. Kinetic energy isn't free in this game, and you will need all the advantage you can get.
Analysis: The physics of this game feel "right". The bounces are predictable; hitting even a 1-pixel slope either way will advance or retard the ball's motion. You will find yourself staring at the screen, hoping the ball will keep bouncing just long enough to hit a tiny declination which will allow it to complete the level.
Advice: When you become very frustrated, don't forget that you are simply trying to get a red ball onto a coloured square. It doesn't ultimately matter.
Of course - I'm kidding. Go get it.
Cheers to Kevin and Ced for suggesting this game!