Even if you did manage to attach something, I doubt it would stay on for long
If it did, it probably wouldn't be very comfortable. I know breeders will sometimes use marker pen on the tail to tell kittens apart (making sure to use a non-toxic brand, I think sharpies are okay, but no personal experience).
Hopefully you'll get used to slight shade or face width differences (etc!) soon. It's possible they have some tiny white markings on their feet or even just toes that may also help!