ASF identification methods

I'm looking for ideas on how I can distinguish my females from one another if the look identical. I want to keep record of pregnancies, biths, and other info, but I have many females that look identical within the same colony.

Maybe a bit of food coloring behind the ears or somewhere difficult to self groom? Just would need to reapply the drop regularly as it fades.
Otherwise you may need to start making sure to cull the ones that look too similar in certain groups to make sure they have unique markings.