I had 4 of these Carpintis just a few months ago. I got them at just over one inch, and grew them to 3-5 inches. The one male quickly became dominant and spent much of his time with black covering much of his body. I believe the black is all about “I’m trying to assert myself” and have others fear me. Once I re-homed the other three the largest male was not black very often any more, except when another fish would start hovering near him. He would then turn black again. I bet your female is just trying to assert herself to her tank mates.