2 of my cockatiels eggs just hatched and they both have yellow down. I am wondering, what can I do so that I can know which chick is the oldest and which one is second and so on?

I don't have any leg bands and I am not sure how I can color the chicks? They move too much. I was thinking of using a sharpie but I recently was a picture of cockatiels with the top of there crest black, and I don't want that for my babies.

Any suggestions are welcome!

What about putting a (tiny ) dap of elmer's glue on one of the legs or something?
Would that work?

For what I understand Elmers glue is not water resistant, so I guess it would come off pretty quick.

Baruch I had the same problem, and I am very curious to hear what other people have to say about this.
My chicks hatched with 2-3 days difference so it was easy to recognise them, the first weeks: there was a visible difference in growth.
But after a few weeks they start catching up and especially two of them were impossible to tell apart!

I thought of colouring the tip of the tail with the purple dye I use for my hair, semi-permanent dyes are vegetable based so I thought they would be safe.
I didn't do it, in the end, because I didn't find any information about whether they are indeed safe, but there must be animal-safe dyes on sale somewhere, after all people pink dye their lap dogs, no? And the colour comes off after a while.

I ended up using a washable marker. I used green for the oldest and brown for the second. I felt so bad when I did it, I felt like I was treating them like a piece of paper. I'm so sensitive!

It's OK to mark them with a washable nontoxic marker. They aren't going to have a bath anytime soon, and if you see the mark fading away you can always renew it. They grow so fast that even a few hours difference in age can make a difference in size, so you might find that you don't actually need to mark them. It's not essential to know who hatched first, so there's no problem if you don't try to keep track of who's oldest. Being human though we usually like to know!

The clutch Jaid was in, the breeder marked a green mark with a Sharpie pen on the cock's feet and red for the hen's feet. I still can see a little faded green mark on his foot, but it's slowly wearing off.