I (sort of) like the basic idea here. Often, I've wanted to acknowledge a comment without leaving a reply - indicating to the comment author that I appreciate/found their comment useful. It certainly would cut down on a lot of comments that go like:
However, there's the obvious problem of interference with "hotness" applied to comment scores. When a score reaches a certain threshold it changes colour, ranging from cold to supernova. Also, if you change the colour range to greens for every vote on a comment for the post author it signifies importance of his vote above other all other votes which isn't always the case.
Most certainly, I wouldn't like to acknowledge all comments I vote for in this manner. I might just find a comment funny rather than particularly useful and I don't think casting my vote on that comment should change the way it looks to other users.
It's a nice-ish idea if it could be implemented in a better way without adding any bloat to the system.