Buggy
The fill-a-pix puzzles are really buggy. A few of them have gone right past the puzzle without ever letting me solve it, and one of the squares in one of the later puzzles simply can't be filled in with any color.
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Mystery Mosaics (Full) appears to be a legitimate app. AppsHunter Legitimacy Score for Mystery Mosaics (Full) is 80/100.
Based on 80 of 100 points across 7 verification signals from App Store and developer data.