I think the title describes all. They will show more obvious when the thumbnails haven't done loading yet. The picture following here will describe it better.

image description

I use a userscript named YouTube Center.

  • May be its nothing but the custom video progress here take a look at this source userstyles.org/styles/95280/… – Vembu Feb 26 '15 at 15:01
  • I don't think so. Those bar still appear when I disable the script. Maybe the like/dislike bar? – Ooker Feb 26 '15 at 15:56

What OP's image shows is a like/dislike bar put there by an extension (like YouTube Center). Since getting likes/dislikes costs API quota, it doesn't load them always and displays a purple bar instead.

YouTube however now has a similar feature where they use a red/grey bar to show how much of the video you have watched. It looks like this.

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