Risking that loud gasp at such an impertinent question, but are there premixed skin tone oil paints beyond the standard single color that the large manufacturers provide?
I typically mix my own, but am aware there are hundreds of people who have a “standard” they follow (Marvin Mattelson for example). Surely, though, someone has come up with a set or series of basic skin tone color paints. Again, not basic colors that can be mixed, but various skin tone premixed.
I have an old tube of Grumbacher oil paint called "flesh". I got it in a bag of oil paints I bought at a tag sale. I have used it for skys (sunsets etc).
I think even the most money hungry paint manufacturers won't stoop to that level. Otherwise they would have a a set of greens called "Tree 1" Tree 2" etc. Or sky blue.
You can mix your own and buy empty tubes and fill them but I think you would be destroying any direct observation freedom you would like to use.
Years ago I had tried John Howard Sanden's Pro Mix for portraiture. You might like it. It's easy to follow his concept. You do not have to buy the pre-mixed kit but if you are just starting out in portraits it's a nice place to start. He does list the colors so you can pre-mix them yourself. The colors are a bit too peachy for me so I mix my own with a simple palette of burnt umber, utlramarine blue, alizarian crimson, titanium white and cad yellow pale. It's a nice palette and works well for skin tones. Hope it helps:-)
Howard Sanden link: