I have seen people put hair scrunches. I also googled it to see what people said. Some recommended nail polis ... on different paws to tell them apart (which might be smelly.) Another recommended nipping the nails on a different paw. I think the best one might be to cut off a different patch of fur on a different part of each kittens body ..... until they are big enough for a kitten break away collar (that you can then use a marker or embroider different color threads on each collar.) Lastly someone recommended buying cheap ribbon and Velcro to make small break away collars.
Awwwwwww adorable little kittens! One of my kittens had black fur and he looks like a panther with his stocky body, super glossy fur and green eyes.
I was so exceedingly worried about him being adopted (black cats are 50% less likely to be adopted in shelters) so I taught all three kittens to high 10 by the time they were 3 months old (I was shooting for high 5 but rewarded any paw raising to begin with .... then eventually got them to stand up and raise both paws to tap my fingers holding the treat above their head.) If you want to go that route I would google how to train cats and start training the moms now (cats learn best from other cats) *It worked! I got my mom to love him enough to keep him as a buddy for her kitten.
That said you can also google how to get the best pictures with black cats. I found a well lit room and white blanket made a world of difference when photographing my baby panther. I also photographed each kitten every few weeks (next to the same object) to track /document their growth.