Incredible~! This submission is super polished and a clever interpretation of the theme. Would not have thought about doing a Splatoon style of game where you fill an empty void. I think the only bit of critique I could give would be about your menus. Your sliders were pretty small and I could barely make out the control layout you had on the right side of the menu. They blended into the background and I honestly would've preferred just having the same color as the other menu elements.
You definitely should develop this game some more, would be awesome to see where you take it if you continue it. Could see all different types of enemies and alternative ways to fill the level with color.
Awesome stuff and congrats on your first GWJ!
- S. "Essay"