Hey, cool concept! The idea of power-washing pixel art with color matching is really clever — it's one of those mechanics that's immediately understandable but has hidden depth.
Going through your questions:
1. Core mechanic: Color-match + swipe to reveal hidden images is inherently satisfying. There's something almost meditative about "cleaning" pixel art. The water limit turns what could be a mindless activity into an actual puzzle, which is smart.
2. Difficulty curve: Heard you already pushed v5 with better water budgets for early levels — that's great dev responsiveness. One thing that helps with difficulty spikes: give players a "preview" glimpse of the full image before they start. It turns the puzzle from guessing into planning.
3. Wrong-color penalty: Re-dirtying neighbors is a really nice risk/reward mechanic. It punishes carelessness without being random. As long as the player can see which color they're about to apply, it feels fair. If they can't see clearly, it feels like a gotcha.
4. Lock mechanic: Forced solve order adds genuine puzzle depth — it's the difference between a coloring book and a puzzle. I'd suggest a subtle visual hint (maybe a small arrow or glow) showing which pixels are unlocked, so players learn the system through play rather than a tutorial wall.
5. What would keep me playing: The "reveal" moment when you finish a level is the hook. I'd lean into that — maybe a gallery of completed images, or let players see a zoomed-out view of their progress. Collectible completion is powerful motivation.
One suggestion: since color matching is the core mechanic, a colorblind accessibility mode would expand your audience significantly. Even a simple icon overlay on each color would work.
For a first game this is very polished. The scope is well-controlled and the core loop is tight. Good luck with development!