Pencil diesnt fade. I use popsicle sticks and oencil for seedling trays. But your jugs look like a sharpie is the right thing. But if you ever do seedling trays try wooden popsicle sticjs and pencil

I've used pencil on those thin plastic tags for seedling trays, but I don't have good luck with popsicle sticks... they get green algae or black stuff on them, and the writing becomes illegible. Maybe I just keep my mix too wet.

I've also started using pencil if I'm hand writing plant labels on zinc tags... or for putting notes of some sort on the back side of the tags.

Thanks!