The Colorado Tag requires you to notch out the sex, month and day of your kill with your knife. You need to do that immediately after your kill. You can sign your name with the end of a lead bullet but a pen does make it better. You do not need to attach it to the carcass until after you have moved it to your camp or vehicle. You would need to carry some string or wire ties with you so that is something else to add to the list.A few folks listed it, but dont forget the pen!!!
I heard a story about someone getting a citation for hunting without one. You cant properly tag your game without it.