Hi, I am having the exact same problem and I think the latest version of ColorfulTabs is conflicting with Tab Mix Plus. If I turn off Tab Mix Plus I get the colors as usual and if I turn it back on again, all the tabs start out colored, but then turn into off-white. In my Firefox this is the same color like when I don't use ColorfulTabs.
The version of ColorfulTabs I had before this update didn't have this problem, so I hope you can fix this anytime soon.
Thanks in advance, greetings,
Daniela