Like many of you, I also have a RepRack filament storage system for all of my precious spools of plastics.
There are many labeling systems for the RepRack, but they are magnetic, and since I am using 16mm plastic tubes, those were the only ones available at my local home center. So, I decided to make cards that would slot onto the rods.
I've included the Fusion 360 file for this design if you need it for a different rod size or different labeling.
If you need a label for another common type of filament, let me know, and I'll add a .still.
I've included two different versions. A slanted label and a flag. The flag is good if you must divide a shelf into multiple filaments. The slanted label is better if you put it at the end of the rack.
No supports are required, but I'd definitely do a two-color print if I were you. I printed mine in PLA since there's never any sun in my 3D printer room, so they'll never get too hot, but you could easily print them in PETG.
I've included a couple of PrusaSlicer files for your slicing convenience. The slanted version will fit two on a standard i3-style printer. They are set up with the PLA label, but just replace the STL with whatever label you want.
I printed mine using Prusament Vanilla White and Prusament Galaxy Black.
Enjoy!