That can be a lot of work! That's why I use the extension "Session Manager" to restore my sessions. Works perfect for me.
I don't have to restart Firefox to see if it works. With me it works instantly. If I check the box and save the change, the tabs immediately become coloured. If I uncheck it and save the change they immediately become off-white again.
Maybe you changed another setting? I uploaded a screenshot and marked the places where you need to go to make the change. Maybe that's easier to understand than my translation...