In 1981, photographer Lynn Goldsmith took a portrait of the musician Prince. It's a pretty standard headshot — it's in black-and-white, and Prince is staring down the camera lens.This was early in his career, when he was still building the pop icon reputation he would have today. And in 1984, shortly after Prince had released Purple Rain, he was chosen to grace the cover of Vanity Fair. The magazine commissioned pop culture icon Andy Warhol to make a portrait of Prince for the cover. He used Lynn Goldsmith's photo, created a silkscreen from it, added some artistic touches, and instead of black-and-white, colored the face purple and set it against a red background. Warhol was paid, Goldsmith was paid, and both were given credit.However, years later, after both Prince and Warhol had passed away, Goldsmith saw her portrait back out in the world again. But this time, the face was orange, and Goldsmith wasn't given money or credit. And what began as a typical question of payment for work, led to a firestorm in the Supreme Court. At the center of it, dozens of questions of what makes art unique. And at what point does a derivative work become transformative? The answer, it seems, has to do less with what art critics think, and more with what the market thinks.Help support Planet Money and get bonus episodes by subscribing to Planet Money+ in Apple Podcasts or at plus.npr.org/planetmoney.