Pretty fun, but a bit too easy to beat (which really cuts down on replayability). After running through the tutorial (cute prompts :D), I was able to beat it on normal and hard on the first run of each. I didn't lose any shields either time. Would adding an endless mode be a possibility? Or perhaps anchored tiles/gray tiles, which either can't be moved, or can't be matched.
Have you experimented with adding some kind of animation/tweening when the rows/columns are shifted? Just moving the squares' colors is kind of confusing, visually.
I liked it overall :)