I can help you here.
I did paint the roof of my own car white.
As its an old car, i used white metal paint with anti rust properties too - usually ppl paint fences with that paint.
Before that car roof was british racing green. Every time i got in the car i feelt like i was in sauna. The heat accumulated by the roof severely radiatet inwards. It was like being 5 cm away from an electric 5kw heater on full blast. Not even an extra layer of heat reflecting insullation tucked between metal and rooflining helped. It was quire severe situation during summers.
Now this problem is solved. No more furnace effect from roof when entering the car. I cannot comment on specific numbers, or the total drop in temperature of the cabin as ive not measured it before and after.
What i can tell you exactly - before painting, roof lining was uncomfortably too warm to hold your hand on. Now its barely warm.
White color works miracles. My next car will be white due to AC economy and comfort considerations.