I played with the free Windows PC version for a while. And it seems fun. Nice job on this so far. It could be something for kids (or any beginning pixel artist) to play around with. It's very intuitive and easy to use. You provide different color options, multiple screens, options to erase a screen, and cycling through different screens. So there's potential here.
Of course, a natural next step would be the option to animate the image by sequencing through the screens at some user-specified rate (instead of cycling one screen at a time). If that option already exists, I missed it. It would seen like a simple feature to add. And being able to make and re-play animations would increase the fun and versatility of the app tremendously.
Beyond that, maybe you could add the option to save images to disk. You could use the screen_save_part() function to capture and save a single drawing screen as a PNG file. Or you could use the ds_grid_read/write functions to save and reload images from ini files. These features might only be practical on Windows versions, but they'd add to the capability of the app.
But it's a good start as it is. And lots more you could do with this.