Very atmospheric! I got a strong "Thomas is Alone" vibe (but I guess that could be said of any rectangle-jumping game, heh)
It's very hard to get an audience to connect emotionally with a simple shape, but this game achieves that!
I will say that I struggled a little with the platforming aspect, especially having to control everything with the left hand. When playing a platformer I prefer to have all the controls on the keyboard, instead of spreading them between mouse and keyboard, which I personally find cumbersome. Seeing as the only mouse control was to hover over the diamond to unlock the door, I feel like this could easily be achieved, but that's just my 2c, not all will agree with me there.
Overall, a solid entry with a touching story. The color shifting and cinematic bits were all spot on! Well done!