I could give you a few recepies to start with for your skin tones, but think about light, mood, and shadows.
A person under a red light at night will present diferent colors than another under a blue clear summer sky.
For the purpoe of practice and exercise here are some suggestions for skin tone mixing colors ( for any medium).
I'll start with the largest quantity.
titanium white --- yellow ochre --- burnt sienna --- ivory black ( note: I use viridian green and alizarium crimson to make black, I don't use black on my oil
titanium white --- cad red --- yellow ochre --- burnt sienna
titanium white --- cad yellow light --- viridian green --- ultramarine blue
titanium white --- yellow ochre --- cad red --- burnt sienna --- ultramarine blue --- ivory black
titanium white --- yellow ochre --- english/ indian red --- cobalt blue --- prussia blue ---ivory black
titanium white --- burnt sienna --- veridian green --- ivory black