I use an orange filter almost always on all of my lenses, including usually group shots of people. With portraiture I have tried light yellow with some success and light green with slightly more success than light yellow.
I then obtained a yellow/green filter and found the most success out of the three filters using Fuji Neopan and Ilford FP4+, which have been my most used portrait films.
However, I only have a 52mm yellow/green filter, which is good for my 35/50/55/105/135 lenses. Sometimes I prefer to use my 85 1.4 or 180 2.8 both of which use a 72mm filter. With these lenses, I use no filter for portraiture.
The yellow green filter works best if photographing someone wearing no lipstick. Yellow is alright, green didnt work too well for me.
The differences between no filter, and all three filters mentioned, are subtle. Generally, I found the Yellow/green filter gives the best rendition of the three and if I am going to use a filter for portraiture work, I use that one.
Mick.
Edit: go to my gallery and look at New Haircut and Different View pictures. The 105mm lens one is with a Y/G filter, the one with the 85mm lens is without a filter. Check out the lips and a few other things, you can see the differences. Same roll of film and obviously same development.