Maybe there is another setting in Tab Mix Plus you need to change or maybe it's another extension that ColorfulTabs is conflicting with.
To check if Tab Mix Plus is still causing the problem you can just disable it and restart Firefox. If that doesn't solve the problem you need to check the other extensions. If it does solve the problem you just have to find out which setting is causing it.
To check extensions I usually disable two or three extensions at a time (and restart Firefox) and see if that solves the problem. If it does I re-enable one at a time (and restart Firefox) until the problem recurs.