Well I would try the puffy fabric paint that comes in the little squeeze bottles but I would test it on a swatch first. If that doesn't work you can try the brush on fabric paint. Joanne's craft store and other craft stores carry the paints. One that comes to mind is tulip fabric paint but there are other brands that are good too. Mary!

I cant think of anything that would work well on yarns..I think the puffy paint need heat to make it puff at least it did when I used it for card making like snowmen noses,.

If it was just for looks I wouldn't mind the puff paint but i have worked with that and it is really hard when it drys, I was needing something that would still be soft, had actually thought about using a permanent marker but am needing white and have never seen a white permanent marker LOL. Thanks for answering.

sounds like a great project ! I will sometimes do script and stuff in faux cross-stitch over crochet, but that wouldn't work with popcorn stitch...

they do have opaque white 'paint markers' at the craft store...

but I would use a flat-tipped brush, either a 'fabric paint' brush (short, stiff bristles) or a white nylon bristle, and a good-quality fabric paint (Jacquard is ideal, so-soft is also good)

in both cases, let dry, toss it in the dryer on high for 10-15 minutes, take it out, touch up any blank spots, and then toss it back in the dryer for a second round.