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