I've been trying a new add-on called Fabtab .
https://addons.mozilla.org/en-US/firefox/addon/12956
It has an interesting idea of taking a snapshot of the top the pages, detect the main color, then set the color of the tab similar to that.
Unfortunately, its compatibility with other add-on is quite low while ColorfulTabs work flawlessly. I hope you could do some research and implement this idea in ColorfulTabs if possible. Thank you.