This is my favorite swatch and it is a great test for transparency/opacity. I love to keep them on a ring, but they were so bulky I couldn't fit any more on the ring. I used a hole punch to fix my existing swatches, and then I remixed the model to make my own so all future tests can be done the right way. 

Print with 100% infill if you want to test transparency. Write the brand and color on the back and store in a binder ring. 

Minor changes from 3dingus original model on thingiverse.