Why not just lost the caps entirely and decorate the inner portion of the rim? Etch the plastic, then fill in with paint and remove the excess like they do with other items(not necessarily yoyos)?
Personally, I like how the caps finish the look of the yoyo. I also like it when the yoyos are marked with the brand and model. That way when my memory fades, I’ll be able to tell at a glance(or read) what the heck everything is.
That reminds me, I think I should go and set up my photobox and start cataloging my stuff.