IM not sure thats a portwine stain....
Just looking at your message and the attached image and Im pretty sure thats not a portwine stain. I dont have any medical experience (unlike loads on here) but having a daughter with a PWS, my understanding is portwine stains are always pink / red or purple in colour given they are caused by the capillaries under the surface that cant constrict and the increase bloodflow gives the PSW its colour. In the case of older PWS the mark can become purple because the bllod folw becomes slower.
I hope somebody can give you a better idea but I would get a second opinion..