For my last few paintings, I've tried to stick to a proper yet fully functional palette for the Ébauche, or first pass.
This palette should consist of paints with low oil absorption (so as not to violate the fat-over-lean guideline), and be opaque to facilitate solid modeling of the objects.
Ideally, the paints should also be fast driers, but that would narrow the selection too far for my purposes; it would certainly not leave me with primary mixers.
Although I have a large number of paints on my full palette - about 75 - applying the low oil and opacity filters narrows the selection down to about 10 - several of which were fairly close; cadmium yellow light and cadmium yellow deep for instance.
The final selection I've been working with has been as follows:
Cadmium Yellow Medium
Hayes Blue (see below)
Hayes Violet (see below)
I had to make a few compromises, as strictly applying the filters would have left me without a few pigments and characteristics I needed.
"Hayes Blue" (yes, that is a little tongue-in-cheek) is a mixture of Ultramarine and Cerulean blues. Ultramarine is a low oil content paint, but is transparent, therefore a poor choice for modeling forms. Cerulean is opaque, but has a higher oil content. So... A mixture of the two. "Hayes Violet" is simply an addition of Indian Red to this mix, resulting in an opaque violet (all others are transparent), which also makes a lovely warm grey when mixed with white.
Mars Black is my other concession. Strictly speaking, all available blacks are high in oil content, but I simply must have one on my palette, so I chose Mars for its opacity. It's awfully warm for my taste, but I glaze over it with ivory black to cool it down. Easy.
Finally, to make them easier to handle and speed up drying a little, I added 1 (one!) drop of walnut alkyd medium to each nut of paint.
So that is my current underpainting palette - with it's requirements, trade-offs and compromises. Although it is inflexible and incomplete in several ways (particularly in terms of temperature), it allows for a solid underpainting that can be easily refined in the final stages of work.