Most of the multi-color coasters I've seen either require glue or performing a color-swap that changes the entire layer's color.
I have designed these so that the letter can be printed and inserted into the coaster during a pause and then is sealed in place by the following layer. The visible portion of the letter fits a void that runs through the coaster's surface and the lip on the letter fits in a lip on the interior of the coaster.
I chose IBM Plex Mono Bold for the font and printed “SISU” in blue and white (Inland PLA+) to honor my late grandfather who did electrical engineering work for IBM and was the son of Finnish immigrants. It is a gift for an aunt.
These should be printed with a 0.2mm layer height (both parts) including a 0.2mm first layer, as both the inserts and the voids are 2mm. It was a nice, clean fit on my Prusa Mini+, but I would caution you if you do not have the dimensions of your printer well tuned. I used a 0.4mm nozzle, but it may work with 0.25mm or 0.6mm (untested). I used random seam positions, but if you want them in a consistent location on your coaster, I suggest printing the inserts with a random seam position to avoid tolerance issues.
The orientation of the parts in the STLs are how they would sit in real-world use orientation. PLEASE NOTE: The coasters should be flipped so the void is on the build plate, which will prompt a warning about overhangs and supports in PrusaSlicer. This is by design. A pause at the start of the 2.2mm layer height (i.e., after the 2mm layer has finished printing) will allow you to press the letters into their corresponding voids. At that point, the overhangs are all supported by the inserted pieces and the 2.2mm layer should seal the inserts into place.
If you have a smaller printer (like a Mini+), you'll only be able to fit 2 coasters at a time (they're just under 90mm in diameter), but larger ones should fit 4 or more. The letters are significantly smaller than the coasters, so you should be able to fit quite a few on the build plate at once.
The holder I printed was the “04_High” model, but if you have more or fewer letters, you can choose another holder model accordingly. Note that while the inserts are a very close fit (to minimize debris accumulation in the coasters) there is a decent bit of clearance around the coasters in the holder which should minimize issues with friction from seams.
The author marked this model as their own original creation.