A lot of Fj40 owners have also used it on the bottom side of their fiberglass roof tops rather than using headliner material.
I've not painted any roofs a dark color, but I can attest to the benefit of a light colored roof. There is a reason that an off road vehicle that was designed and built before A/C was common came with a white roof from the factory. Many users have thought they knew better and soon painted the roof white again. I can tell you this much, if I have the all the vents open it remains the same temperature inside as it is outside, and most of the time it's the temperature it is in the shade. This all despite there being no significant R value to the 1/4" thick roof.
x100 on keeping it a light color. The top of my Ambulance is going to stay white... the only reason it'll look darker is when there are solar panels mounted on the top of it... and I may even insulate between the solar panels and the roof (but that's a topic for a different thread).