My Color Picks
I use Maimeri Blu watercolor paints in my painting classes. I recommend to my students a set of ten colors. A warm and cool red, a warm and cool yellow, a warm and cool blue, two neutrals plus a green and a purple. With this combination you can mix tons of colors.
The colors I recommend are:
Primary Red Magenta
Sap Green (if available)
Primary Blue Cyan
Permanent Violet Bluish
I have these laid out in my palette with the neutrals on the left. Starting with Payne's Grey on the lower left (then I have one extra neutral Sepia, rarely used) and Burnt Sienna. Across the top is Primary Red Magenta, Crimson Lake, Indian Yellow, Lemon Yellow. Down the right side starting at the top is a little bit of Permanent Green Deep (an optional green if Sap is not available), Sap Green, Primary Blue Cyan, an extra color Cobalt Blue, Ultramarine Blue, Permanent Violet Bluish.
It's great to have the basic ten and then add in a few of your own favorites.
If you want to go with a more limited palette, I would recommend:
Paynes Grey, Burnt Siena, Crimson Lake, Indian Yellow, Sap Green and Ultramarine Blue.