Recently, I have been amusing myself with the interesting puzzles provided by the excellent Flow Free puzzle app.
Since I couldn't find a way to bookmark my favourite puzzles, I took up a habit of screenshotting the ones I liked especially, for one reason or another. This question is an excerpt from that screenshot collection.
If you are unfamiliar with Flow Free, it's a puzzle with very simple mechanics. There's a square grid, peppered with pairs of different coloured (and optionally, lettered for the colour blind) squares, which you are supposed to connect pairwise by colouring adjacent squares, without the paths ever crossing. The resulting tangle of paths will always be the unique solution, and it will always completely fill the square grid. (There are some exceptions and variations to these rules, but all the ones I'm posting here will be of this "pure" variety.)
In so many fewer words, you are supposed to turn this
The puzzles below are not of the easiest ones, oftentimes quite the opposite actually, so if you get stuck, the app has a lot of easier ones available.
The app itself has hundreds (more likely, thousands) of puzzles like this, and it's free (as in ad-funded). Posting this is probably pretty deep in the grey area regarding both intellectual property rights and the PSE guidelines. However, since I'm pretty sure PSE likes great puzzles, and also that the app providers don't mind unpaid endorsements, I figured I'll just go ahead. Please drop a comment if you disagree.