I am a 26 year old with a facial PWS.
I have looked into a medical tatooing, but what they do not tell you is that after time your birthmark will come back because the blood in the capullaries will start to swell and then bleed into the tattoo making the area look worse then it did before they did the tatoo.
Yes, the first year or two (depending on the extent of your PWS) it does look great but within five years give or take you would be able to see your birthmark again because the pigment in the color of the tattoo will be vary dark and it will first start looking like a brouse and then the birthmark will come back in bloches. If you google before and after pictures from it I am sure you can find the ones I saw. Look for the pictures five years and ten years later though because that is what it will end up looking like after the birthmark takes over the tattoo.
Just out of curious how old are you? The reason I ask is because when I was in my early 20's my birthmark started to grow. Get thick and I ended up having to get plastic surgery to remove some of my birthmark and it has already started to come back. Some doctors claim it is related to hormons but until they find something to make it stop growing anything is possiable.
I would strongly recommend doing your research before considering the tattoo. I started laser treatment when I was five and it still is not gone. I have excepted that unless technology gets better I will live with this birthmark for the rest of my life.
If you have any questions let me know my email address is linked to my account.
Have a great day.