You move a coloured ball, which is submerged under the water filling half the play area. Other balls drop into the water at a steady rate, occasionally bouncing off the plank floating in the centre of the screen. If you touch a like-coloured ball, you'll collect it for points; if you hold down the mouse button as you touch a different coloured ball, it will bounce off but you'll take on its colour. If a ball falls all the way to bottom of the screen, the water level rises, and it's game over when it overflows.
Things start fairly slowly, with just two colours making things readily manageable. But once a third is introduced, my brain struggled to adjust with juggling the necessary colours. It all gets a bit too frantic. Maybe you'll fare better?