From the Super Mario Wiki
The Prisma Fountain is a major tourist attraction and one of the most vital areas in Paper Mario: Color Splash. It is located in the middle of Port Prisma, in the red district. The six Big Paint Stars were originally here, before being thrown away by Bowser. Once one is retrieved from one of the Koopalings, it briefly shows what happened before Mario arrived in Prism Island at the beginning of the game. Finally, once all are collected, it reveals the true story behind the game.
Mixing the paint created by the Big Paint Stars will result in black paint getting created.