First off, read this thread:
There might be some confusion in there, because one post was deleted. Someone had advertsied their services in this area and that's a no-no around here. Also, the long post with all the information in it really compares laser and tattoo, and that's not what you're asking about. Also, I see that you are not asking about a flesh-colored tattoo, but one that matches the color as it is now.
Have you tried any laser treatments? Also, when you say "severe PWS" do you know the level of severity?
My recommendation would be to ask a physician that is familiar with treating PWS, especially severe ones.
And, for the record, I'm not against tattooing... I have one. I got it for my sister's 40th birthday (she was turning 40 and I was celebrating because I wasn't turning 40) and all of us were going to go get tattoos with her. My brother and I, some nieces and nephews, and my sister. Guess who chickened out? Yep, my sister.