This should have been a feature since day 1. After I made my ~8th notebook, it got very very difficult to easily locate the notebook that I was looking for. Being able to change the colour/pattern of a notebook would be great, but being able to add a "front cover" (a photograph) would be even better.
Sorry that this is an old thread (Google brought me here). Disappointed to see that this is not possible in Evernote. Learning how to properly use styles in Pages (OS X) really improved my workflow. Hope it comes to Evernote soon.