Drag down on the creature and release to fling it into the air. The aim is to make it land in the bucket, filling it with paint in the process. If you miss, or if you hit one of the spiked baddies on later levels, your score goes down - reach zero and it's game over.
A lovely concept, although frustratingly difficult! See how far you can get (personally I couldn't even get past level 3)
The game source code is for sale on Code Canyon, but you can also play it there by clicking the Live Preview link.