I understand the idea of voting for a great comment, but why does the comment's score change colors at five-ish up votes? Comment votes don't seem to give reputation, or anything else, for being great.
All the votes do is help to visually identify good or insightful comments. You are correct, the color of the vote count does change at (IIRC) around 5 upvotes, and then again around 10. The specific color is tied to the site you're on; for example, the color becomes more orange on Stack Overflow.
Also, when the page initially loads, there will be at most 5 comments below each post. The rest of the comments can be expanded out, but the initial comments are the five highest-scoring.
Other than that, the score serves no purpose. However, I will say that even though I don't get reputation from a comment vote, I personally like to see that someone liked my comment and thought it was good or insightful.
For more information, see http://blog.stackoverflow.com/2009/04/comments-top-n-shown/
I still think there should be a badge, even a bronze one, for getting 10 upvotes on a comment.
Expanding on Kyle, the primary purpose of them was to get the best comments loaded on the page for all requests (instead of via AJAX) - so they can be indexed by search engines/visible to all users.