Is reflective solarguard roof paint cost effective

After a few years on a colourbod roof how well does reflective paint work. I imagine with deterioration and dust it could well be dramatically hotter inside a house than when intially applied.

I have a colourbond roof with a foil sheeting as insulation below the colourbond roof. I am not aware of the R value but my guess is from the house I am in, it is a standard product rating for Brisbane.

Would it be redundant to paint the roof with solarguard to enhance the reflective component of the roof.

Would the tempetaure drop be noticable given the existing insualtion already there?
I have been asked about this type of paint numerous times,however I cannot say if indeed it does work or is just sales hype.

It it indeed works,why don't they have it in a black or dark blue ?

They only have in lighter colors and that is what makes me believe that as long as you have a light colored roof,you will have the benefits of a cooler home.;)