I don’t usually bother to mark my thread palettes with symbols and/or color numbers because I stitch really simple things. When a design calls for something like what’s pictured above, it’s pretty obvious what goes where on the chart… but when I kit up something that has a few different shades of one color, I mark the backs:
I used a piece of tape and a Sharpie marker… when I’m done stitching, I’ll just peel the tape off. I’ve tried masking tape, shipping tape, and Scotch tape on these, and the type of plastic I use to make these threadholders allows for easy peeling with no residue.
Oh! I made some for Halloween… you can find them at the webshop: