|Split primary palette - almost|
Here is the almost classic split primary palette - two yellows (Cadmium Yellow Light and Quinacridone Gold) and two reds (Cadmium Red Scarlet and Anthraquinoid Red), but only one blue, which can be moderated with Phthalo Green to make a cool blue.
Phthalo Green is also useful to make grassy and other greens mixed with the yellows or blue, and with Pyrrol Crimson or Anthraquinoid Red (or another permanent Alizarin option) to make a fabulous black.
There is also Quinacridone Rose, four earth pigments and a mixed grey (which I call Jane's Grey :-) for convenience. This palette could be used for almost anything from landscapes to flowers to people.
However, I found I didn't need the lemon yellow, and replaced it with a mid yellow (Hansa Yellow Medium), and replaced the two reds with Permanent Red Deep and Quinacridone Violet. I can still make a bright fire engine red or an orange-red by adding quinacridone gold, and pinks using the two red/violets with water. I added Cerulean to save time when painting skies and water, removed Indian Red because it is easy to mix and replaced phthalo green with two useful greens, since I paint a lot of landscapes - Undersea Green and the highly granulating Jadeite. Goethite, a granulating raw sienna/yellow ochre substitute and Buff Titanium (great for beaches, shells and corals) make up the rest of the palette, along with my premixed Bistre or Jane's Grey. Once again, it can be used for just about anything. There is no right or wrong with a choice of colours, just a lot of trial and error, and gradually working out what works the best for your purposes. What are your choices?
Happy plein air painting!