I've created these musical clef key fobs that are smooth on both sides. They are designed for dual-color printing in black and white and don't require a Multi-Material Unit (MMU) or adhesives. In the design, the white background is separated from the key tag within the *.3mf model files and consists of a single layer (0.2mm).

If you have a multi-color printer, the process is straightforward. If not, you can follow these steps, as I successfully used PLA

• Ensure your print bed is clean.
• Load the white filament.
• Begin by printing the white background (c/f/g-clef_white_*.gcode) and keep it on the print bed! The black part will be printed on top. Don't worry, pre-print calibration is not affected if you leave the white part on the print bed.

• Pre-heat the bed to PLA temperature (do not allow it to cool down!).
• Switch the filament to black.
• Print the corresponding key_tag_*.gcode file, either with or without ironing, as indicated by the file name.
• Ideally, the nozzle should not scratch the white piece and should fill any empty holes.

If executed correctly, both colors will seamlessly come together. Enjoy your creation!

 Edit: I've also added a two-sided version with both clefs, F and G - as a greeting to all the piano players out there.