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I remember there being two girls, in their early twenties, maybe. They are from Earth, but somehow (drawing a blank) end up in another world. In a Wizard of Oz style, they must reach the realm's giant castle, but it's possible even that won't get them home.
One of the girls is approached by a local woman who offers them a means to make money for their journey: become a painted lady. These are highly treasured (and legally protected) courtesans who are painted with intricate blue "tattoos" across the whole of their bodies. There is a small vial of potion offered which allows for complete conscious choice, but wipes several hours of memory. She's told if she takes this role, the vial will help both sides - many political/royal figures hire the courtesans and pour out whatever moral dilemma, (etc.) they are dealing with.
She takes the markings, goes off on her first job, and decides NOT to drink it, so she will be able to remember and decide if she's really okay with doing this. If I'm remembering correctly, the painted ladies wear a minimal amount of clothing, a sheer wrap/dress.
The tattoo pattern was swirled, and I remember it being described as winding around her arm like ivy.
I think the cover of my edition had the two girls in "modern" clothes at the bottom/foreground, looking towards a distant castle in a mountainside. I don't think I ever finished the book (it was the 80's, I was a teen, and lived in walking distance of THREE used book stores, so… I tended to buy a lot of books.)