ok, this is one of the more confusing things about Swellinfo and is explained in the <a href="http://www.swellinfo.com/tutorials/tutorials.html">tutorials</a>.
This is actually going to change in the near future, with a pretty substantial upgrade to the timeline graphs, but here's the explanation.
The colors on the surf plots, correspond to the dominant swell category. So, on the surfplots, click on "view details" button (bottom left). You will see all of the concurrent swells categorized by color. The color of the timeline graph above matches to the dominant swell in the swell category. Make sense?
PS. The next upgrade is going to be killer. This will happen sometime in February, maybe within a couple weeks.