The model is currently very finicky. It may not make certain parts of the body dark in that case you may have to specify dark breasts, dark penis, etc.

This works as a general headswap model, but it was mostly focused on female bodies. It may also have the issue of making the head more masculine, so you can try putting some things into negatives to stop that. I suggest using a black choker to try and cover the seam.

V2: I changed the training data to try and make the effect more consistent, but it can still have artifacts around the neck.