Is there a particular reason you don't want to use photographs?

It's possible to paint realistic skin textures using primary colors but it usually takes most people years of practice to get it to an acceptable level. Scott Spencer has a section in one of his books that shows how to paint/airbrush realistic skin on a 3D model in z-brush just using primary colors (red, blue, yellow), and various stroke techniques. But that's for z-brush, so it would require some creative adaptation for other software.

Most people in Poserdom use hi-res photos to create skin, and add various details, shadows, highlights, etc, where necessary, because it's easier than trying to paint real skin from scratch. If you're going for a more painterly look, then painting from scratch might be a better option.