Technical pen and digital colour
This is a Rotring-pen doodle I did years ago. I just coloured it in Photoshop (and not very well, really).
It's difficult to see what's going on in this image, but if you clicky-clicky on this thumbnail you'll get a look at an inflated version (approx. 150KB, opens in a new window). It'll still be difficult to see what's going on, but at least it will be bigger.