Grayscale Sweet game
Nice concept and nice levels. The game looks proffessional and the levels are gradually getting harder. But then on some of the few last stages it gets easier again, why?
As Tage7 said, add some prize to the game for completing the game, a few new stages or another game mode, not some cheap pictures.
Something that might make it look better is if you add borders (hope you know what I mean).
Try to make it more original so it sticks out from the rest of the endless puzzle games. There are a lot of puzzle games that have different concepts, but they are similar in style in a way. Sorry I can't say what to do to make it original. I'm sure you'll come up with something :P