Chick sized leg bands...easy to apply, come in lots of colors.
Can get from poultry supply or even EBAY! Ive heard people doing nail polish but I dunno...I dont want those kinda fumes around my babies. Ive done markers and food color in a pinch...but you forger to remark, it wears off and then you just have an older bird that looks like the rest.
Nail polish will be OK as long as you apply only a small dab, very quickly, in a well ventilated area and then whisk the chick away from the area you had the bottle open (also don't use red or pink... the other chickens will pick at it, so will the one wearing it, for that matter).
Remember, chickens (birds in general) breath differently than we do, literally completely filling and completely emptying their lungs with every breath, which makes them extremely susceptable to airborn toxins.
Rubber bands work good--- hair bands, tiny, colored "ouchless" type. You can get them in 3 sizes, many colors in one package at walmart for under $4.
Just brought home some one week chicks . The seller put some red nail polish on one of each chicks legs. I think they did it to mark different breeds they had. Guess they didn't know what chickens do when they see red What do I do? Surely I can not use polish remover to get it off! Could really use some suggestions.