A magnetic Thumbs Up! I use it for the dishwasher to indicate clean or dirty dishes or just as a fridge magnet. Many uses of course.

Magnet used is a 6mm x 3mm available on eBay and many other sites. Two magnet holes are recessed on the underside just enough to not scratch surfaces when fully seated. Bevels on top and bottom edges as shown in pictures to help remove the magnet from the surface.

I used ColorPrint by Prusa3d to do the color layer change automatically. (Almost automatic anyway) :). http://prusaprinters.org/color-print/

Enjoy!