I don't know a way to do it inside Poser. I've done a couple of images like that, but in combination with Paint Shop Pro.

Lilith in Black and White

Grave Mistake!

In Poser I used all white lights.  After rendering I took the image into PSP, made a copy of it, and  turned the original into a grey  scale image.  On the copy I painstakingly used the lasso selection tool set with a 2 or 3 pixel feather, and went carefully around the lips and hair.  Once I was done, I copied my selection and overlayed it on the grey scale image.  In order to make sure that they matched exactly I changed the mode on the coloured lips and hair to "difference."  Once they were matched up exactly I changed the mode back to "normal" and then merged the 2 layers.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi