Turing Turns is an engrossing puzzle game: guide a ray from the origin to target by setting block colours to turn it in the right direction.
To get to grips with this game, work through the first four levels which comprise a tutorial. There are 40 designed levels which get progressively more difficult. When you've managed those - or want a break from them - try out the randomly generated level system, there are several different types. If you are colourblind, make sure to switch on colourblind mode.