As I understand Purpura is normal after pulsed dye laser treatment but my child had done grey spots instead, why? (photo)

The best laser for childhood facial vascular lesions is usually the Pulsed Dye Laser.

Unfortunately, the typical skin reaction is one of "Pupura" the medical word for purple

skin bruising, as you've previously experienced.  You've probably already gotten over 

the shock of seeing you child's face look so "beaten up", but have seen the bruising disappear in 8-10 days with some fading of the birthmark (thankfully!).  Grey spots may mean a lesser reaction to the laser, as well as a greater than expected reaction.  I'm sure the doctor performing the treatment watched the reaction of the skin carefully after each pulse and adjusted the energy levels based on the response of the tissue at that time.

BUT, this is NOT a case to second guess, as blistering and deep tissue damage may lead to scarring.  So, I strongly advise you to check with the treating doctor.  Sometimes a two minute reassuring phone call can lead to a better night's sleep for everyone.  Good luck with your beautiful baby.

Yes, grey spots can be seen with the PDL, and they heal up in time. The important thing is follow up- especially if the skin peels or blisters develop. Possibly a review the next day by your Specialist is the correct thing to do. RegardsDr Davin Lim Laser Dermatologist.