Instructions are deceiving...
... as it states that colors are removed in groups of four, yet it doesn't mean groups of four. A line of four of the same color would be a considered a group of four, but this only deletes them when they are in a group of at least 4 blocks whose dimensions are at least 2x2. Many times I dropped bricks in spots that made huge groups of color, but it only cleared a small square out of the middle, or didn't clear any at all. Either you could redefine your block-clearing scheme, or at least be more descriptive about how it actually works.
As far as audio/visual, music gets annoying at times as it's VERY repetitive, but at least you included a sound control. The graphics are pretty standard for this sort of puzzle game, but some of the color combinations were absolutely terrible. The red and purple scheme was very hard to differentiate at times, and the yellow and gray scheme was awful on top of the already gray background. Not all people have perfect color vision so those real close color combos can instantly take away from playability.
I wasn't a fan of how the sweeper and the aforementioned block clearing scheme worked, but that's more of a preference than an objective review. Mentioning again, however, the instructions should be changed to more accurately reflect how the blocks get cleared. Also, the ability to restart or return to main menu from the pause screen would be a helpful feature if a puzzle quickly becomes unmanageable and players want to restart.