In his book, "On the Training of Painters," published by and available from the
ASCR (American Society of Classical Realism), Richard Lack lists the following "outdoor palette," which I use as a simple plein air palette. With it you can hit an incredible range, and with it I think you can record enough color information to do a plein-air portrait color study.
It's based on Emil Grippe's palette:
cadmium lemon (or light)
cadmium yellow medium
cadmium orange
cadmium red deep
ultramarine blue
thio violet (Grumbacher, a transparent quacridone violet)
thalo blue
mixed white