I can't say for sure, but everything I read says "stress stripes" or "breeding stripes" (which is likely a stressful time)...who knows. I had a group of 5 females in a 30g at one point and sometimes I'd notice them on the subordinates if the dominant decided to strut her stuff, and they were more prominent when first introduced but faded away with time as they colored up.
But I do agree though, that it's better to add more females altogether at the same time if at all possible. I did it in waves, and it noticeably caused issues in the established hierarchy.