Have you ever filled in a sheet of graph paper with tiny designs, square by square, just for the sheer zen of it? That's kind of what playing Pixel Flood by No Hoy Banda Games is like, except it uses your Android device instead of paper. Pixel Flood's basic mechanic fits squarely into the "color fill" genre, where your goal is to make the entire playing field one color. You can change colors by selecting any of the buttons at the bottom of the screen. The trick is that contiguous regions of a color all change together. This means that once you merge with that huge green area, you can change the colors of any squares that touch that area. You're given a set number of color changes to achieve your goal. While there are many other games that work like this, few do it as well as Pixel Flood.
The most obvious element that sets Pixel Flood out is its charming pixel graphics. The whole game has a wonderfully retro aesthetic that takes you right back to the days when sixteen colors was considered state-of-the-art. There's a lot of variety in the blocks you encounter, too: in some of the level themes, the blocks have little faces, and in others they're all various shades of blue. The puzzles themselves also provide a lot of variety, challenge, and quirky art: this isn't simply 40 randomized levels. You have to plan your strategy ahead of time, and you can learn from your mistakes to improve your game. There's an Infinite Mode that does let you play a random field, if that's your thing. And even the square-shaped levels can have some art to the arrangement of their colors. (Everything's better with smiley faces, and this game is no exception.) Pixel Flood is one of those simple games that's implemented so well that it's a wonder it hasn't come around before.
Update