There are a few different limited palette color combinations I've encountered when creating a duo tone (closed grisaille)

I find it easiest to start the drawing with a raw umber (or burnt sienna, ultramarine blue and white) as an oil sketch on grey ground. Sometimes, if I want to create a composition dominated by warmth, I'll create this sketch in transparent red oxide. Raw umber is cooler so if I need those cool green tones to show up, raw umber is the color of choice.

Then I can build the lit side flesh using a limited palette. Here are some examples of simple pigment combinations that provide a useful temperature range:
- Cadmium red and cad yellow with a little black
- Cadmium red with white only letting the grey show through
- Cadmium Orange, Cad Green Pale, White and a dash of Scarlet 

Later, the intense color can be added. It's useful to create a mini color study of paint directly from the palette (unmixed) to help me see how far I can take it and study the temperature shifts.