This little gem of a game has been around to download from the Apple App Store since 2013. I was drawn to it as the premise is very much like a tile slide puzzle game you may have had when you were a kid, when you have to slide the tiles in the correct order to get the pattern (such as an animal) underneath.
On:Off was developed by French team 1Button. It seems very simple at first, as it looks as though you just need to match the colors up with the color indicators (for example, you put the blue tiles beside where the blue lines are). However, the difficulty levels of Hard, Harder and Hardest rather than Easy, Medium and Hard, should give you an idea of the difficulty level of the game! 
The game costs $1.99, and if you get stuck on a level, you can purchase a 5 key pack for $0.99. Personally, I would advise against buying the keys as I think it’s much more fun to try and work it out yourself.
The game also tracks the number of seconds you take to complete the level, so you can go back to beat your score and try and get the solution in a shorter amount of time.
The sound effects are retro but effective, and there is no musical score. It’s probably just as well, given that you have to concentrate quite hard. My personal peak was level 4-17, as you can see from the screenshot below. It started to get a bit more difficult from there!
Overall, the game gives your brain a great workout, and really lives up to 1Button’s tagline to “Turn on your brain!” Similar to a cryptic crossword, the deceptively simple interface and color scheme conceals a true challenge.
The app is available to download for $1.99 on iPhone and iPad.
