Not sure if this is a new feature of 10.7 DP4 or if it's been there, but I cannot figure out why the dates are either grey or purple on the right. Any ideas? I did an in-place upgrade from 10.6.7 to 10.7 DP4.
Just guessing - but maybe it's local vs. external backups?
The purple "tick marks" indicate Time Machine backups on an external volume (external HD, Time Capsule, etc.)
If you have a portable Mac, the white "tick marks" indicate Local Snapshots on your startup disk.
Like normal Time Machine backups, Local snapshots are usually made hourly. If your Time Machine volume is available, both are normally done at the same time, so you'll see adjacent purple and white tick marks with the same time. If the Time Machine volume isn't available, Local snapshots will continue on the same schedule, so for that period you'll only see white ones.