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.
I use a userscript named YouTube Center.
YouTube – Meaning of Colors Bar Under Clips
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If you are using Windows(*) you can mute an application. In your case, you could mute the browser you are using to watch the videos.
I don't know if this feature is present in other OSs and how it works.
(*): I'm using Vista
One way or another the bandwidth will be throttled by either your Internet Service Provider and/or the site itself.
Imagine everyone with your current speed (or higher) all trying to load videos at once!
Best Answer
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.