The white parts are printed separately and glued on. The personalized names on the covers, I used tinkercad. I imported a “Mario” font and then made it protrude slightly. It should be less than the layer height, but deep enough to provide solid color.

Then in bambu studio, color the protruding letters. When sliced, it’s just a smooth surface.

I learned the technique from a post in bambulab but can’t seem to find it anymore.