Lots of good information and insight here. This is such a subjective topic. My only addition to the discussion can best be understood from reading this blog article I recently wrote.
Colorfully Setting The Mood. I don't look at using color as an enhancement to my art as much as I look at its use as a powerful mood setting and storytelling tool. My comic is more humorous than serious but my coloring is focused on adding a subtle tone to the underlying story and satire. It's a lot of extra work which I hope adds an additional dimension to the comic even though it's meant to be transparent to the reader. It just sets a mood.