My "Only One", was to have only one color in the world. Depending on the current selected color, the properties of the world would change. I wanted to add some more interesting puzzles where you would need to time switching between different colors, but didn't have enough time to figure out a good design and implement it :p
This is a link to my game if anyone is interested in checking it out: https://itch.io/jam/gmtk-2019/rate/461706
Also, apparently my game doesn't even work. I've tested it on Chrome and it seems fine to me. Would appreciate it if anyone can confirm if it's not working for them too.