Thanks for the comments, everyone!
I agree that increased contrast/outlines would help, and definitely wanted to change the colour of the piece you're currently moving. I got sick and ran out of time, unfortunately.
I also went too subtle with the theme: you were meant to play as an A.I. gaining more *self-awareness*. The text and tangram shapes in the later levels hint at what you're meant to uncover about your true purpose, but there's not a lot to go on, especially if you don't make it to the end.
@jimbly if it faded to black, congratulations, you beat it! I just designed levels based on complexity; I wasn't particularly concerned if it would have multiple or just a single solution.