Fade old tabs with age
Fade inactive tabs with age, like Dao's Aging Tabs (https://addons.mozilla.org/en-US/firefox/addon/3542), or else try and be compatible (if possible!)
Isaac Fig Newton commented
Pure LOVE club commented
Fade would obviously be OPTIONAL (disable the feature if you dislike it)... assuming dev integrates this awesome potential
No. I have tab groups that have been opened since before I installed TK, and I've grown fond of (and used to) their coloring. I dislike the idea of having them fade.