Why are some backups grey and others purple in Time Machine in Lion?

I started Time Machine for the first time in Lion (you know, to make sure everything was working properly), and I noticed something odd – on the timeline on the right side of the screen, some backups were purple and others were grey. There wasn't an apparent difference between the purples and greys to me, but I feel like I'm missing something obvious.

I just recently noticed this, too, and discovered this Apple KB article: OS X Lion: About Time Machine's "local snapshots" on portable Macs

When you enter the Time Machine browser (used to restore data), local snapshots will appear on the timeline along with regular backups distinguished by different colors. Gray tick marks represent local snapshots and pink tick marks represent backups stored on your external backup disk or Time Capsule. Note: Pink tick marks will be dimmed if your portable computer is not connected to your external backup disk or Time Capsule.

Emphasis mine.