I'm trying to figure out a way to mark hens as they sit in the nestboxes. I'd like to be able to mark faces or backs, whichever is presented to me. The plan is to go out once every hour for a few days and mark the hens in the boxes. At the end of a week or so, cull those without marks--so I'd like it to last for over a week before wearing off. Different colors would be a major plus, so I could use a new color every day and get a sense of my best layers. Any ideas?