You can use some dye on their nails, just be sure its food grade or at least, non poisonous. Be sure you renew it now and then when needed. It may be enough with one nail. So you can use same dye if so is, but in different postions.
I will not put strings around their neck. I was bit nervous about putting ribbons around their necks at this age. Is it ok to put a sharpie on a back paw?
Sorry, Im not sure what a sharpie is.