It's a game, where your playground is a piece of abstract art. Yours art. Screen is a canvas, mouse is a paintbrush, and paint... Well, paint is a cloud of particles that are very liberated. But controllable. Mostly. Not to mention that canvas sometimes also takes active role, changing colours that you already have drawn.
There is no objective measure of art value to rate a result of struggle between you, paint and canvas. But we can measure some things, like, well, how RED is it. So, paint it red. Paint is as red as possible and as fast as possible to unlock levels and gain in-game medals. And enjoy the art that you are making yourself.
Control paint with mouse movements, painting canvas as red, on average, as possible. Paint behaviour is different on each level. Double click resets the level. Space to pause and admire.