We use dye dots on the head, then bands sizes 4-6. You have to keep changing them as they grow. I really like bandettes, but hard to find in the smaller sizes.
Sometimes if we have to use something on hand, zip-ties.
Recently had 2 eggs hatching from different breeding pairs. One of the pairs I use green to mark. I was worried that the chicks would hatch while I was gone, and I wouldn't know who was who for my records. So when one pipped, I took a syringe, and peeled back a little bit of shell and dropped in some food coloring away from the chicks face.
This is what I got when I got home.