If the problem persists and the sealer does not cure to its normal charcoal black dried color, the manufacturer should be consulted. It is possibly due to higher clay and filler content in the sealer itself.  If the problem is temporary i.e. after a few days in full sun it will cure to its normal color, the initial graying is due to either sealer curing under shaded areas or the surface containing too much moisture. Temporary graying can also be eliminated through the use of specialty additive that helps sealer dry faster and at a uniform rate.