Zip ties work well although you need to make sure that the loose end is trimmed close so they can't grab it and pull it tighter. You also need to check them frequently so they don't become too tight as their legs grow.
I don't think I would use a bandaid ~ too easy to pull off and swallow.
You could try putting a dot of food coloring on their feathers ~ a different color for each one. You would have to refresh it as they molt. Unless it is permanent color it may wash out when they get into water.
You can try painting toe nails, but that will likely wear off.
I'd just see if you can find some colored zip ties at HD/Lowe's type of place and just put them on, cut off the ends and check them daily. When they start to get snug, cut them off and put a new one on.
I'm going to use colored zip ties (Pink & Blue... Those are their favorite colors... Don't ask how I know that!
). I can easily tell them apart right now, though. Waddles is smaller, and has a pinker beak, and Puddles is bigger, and her beak is more orange.
A website I also go on (Live ducks. You can google it) had 2 pekins like me, and they couldn't tell them apart, so when they took their first bands off, they put a mark with a marker on one of their legs, so they could tell them apart. I'm not sure if that would be toxic though?!