Need help identifying kittens all the same color

The stray cat gave birth a few days ago and four of the kittens are all 100% black. Being that they're still so young, they all look identical to me and I have no way to tell them apart. I keep noticing that there always seems to be one that acts different than the others and it's a bit worrisome. I can't tell if it's always the same kitten or not.

Is there a way to mark each kitten to help me identify them? I don't think it's a good idea to tie anything around their neck, body, legs or tail. So the only other thing I can think of is to somehow mark their fur, preferably on their tail since the mother doesn't clean/lick it that much, if ever. But since the kittens are black, I don't know what to use to mark their tails. Paint and white out could be toxic if ingested. The only other thing I can think of would be Elmer's glue since it's labelled as non-toxic. Does anyone have any better suggestions?

Well I do- it's actually something that comes up often. Personally I use the paper wristbands that they give out at concerts (usually sold in office supply stores). Then I either color on each one and tell the kittens apart that way (baby 1 is red, baby 2 is blue, ect) or I just write each kittens name on there. Though, I actually find the colors to be easier to tell them apart at a distance. 

If you go this route be sure you can get at least one finger between the wristband and the kittens neck. You will have to cut them off and replace them as they grow. And, as they start moving around more make sure you can get two fingers in between. 

I know someone else who uses colored kids' hair elastics. These also work fine just make sure they aren't too tight. As they grow you can switch to adult hair elastics. 

Whatever method you choose just be consistent and keep a close eye that whatever is around their neck doesn't get too tight or too loose. Good luck! 

 You can mark them with food coloring,  or paint a claw with nailpolish ,  different toes on different kittens or use different colors. 

The other thing is using little collars of some sort,  the little paper wristbands that EB24 mentioned are actually a good idea.   because if you use yarn or hair ties or something  you have to be careful the mom cat does not get bothered by them and pull at them to tear them off.  and could pull too hard and hurt the kitten.