I did one last year, although the fsc got it very clean the draining properties of the material (ie it lets water through it onto to concrete base and runs to the lowast level) a buff coulor, filter the dedris and let a dark 8 inch wide line of dirt embedded in the material along the lowest part of the fsc passes.
This was very difficult to get out it took hours of rincing and even the I could still see it. After a couple of months the weather naturally blended it all back in.
This was done for a contractor to resael the surface (don't ask its a very special seal that lets the water drain through) in the end he didn't re sael it as this would lock the dark stains in