I like the fact that you can jump around from easy to medium to insane. However, I could not tackle the harder levels until I had sufficient practice on the easy & medium levels.
ColorTrek is extremely polished with outstanding graphics and a crisp user interface. I wish there was the option to draw big X's on dead ends. But that's a minor complaint. The Leaderboard feature should also be used - just to see who else in the world is playing. I'm sure the person who tackles all 4300 levels would like to see their name at the top for bragging rights.
Addendum: Markus Ansmann emailed me about the development. He and his friend Martin are a 2-man band with loads of puzzle ideas for the future. According to Markus: all the levels were built by the computer, which he programmed, so they get to solve them too.
http://colortrek.vandenmars.com/ Here, you can play some of the levels on your laptop.