A 2" swirly extrusion indicator with built in filament change (M600 - without a MMU). I recommend Black for the first color, then red, orange, or anything bright. Two 8mm magnets are glued together, then to the back (in a magnet recess). Clean out the recess so the magnets sit flat before you apply glue.

Print instructions

Use your favorite black PLA and something bright. 2 8x3mm magnets and some super-glue is all it takes.

Don't blame me if you're barking like a dog after staring at it too long. It can be hypnotic, you've been warned lol.