Recently, in one of my "Beginning Watercolor" sessions, one of my students asked "How do you make brown?" Without thinking a response, I replied..."I never try to make brown". Which isn't exactly true, once I put some thought into it. When painting a subject that will include "browns," I'm not really thinking "brown", but rather, "warm" neutrals (as opposed to grey, "cool" neutrals).
My "browns" are usually mixed using the other colors from my primary palette, providing a color harmony and unity in a painting, but yet enough variety to maintain visual interest. Notice in the two paintings [in progress] how the warm and cool neutrals play a supporting role to the star colors in the birds. Though I have not intentionally painted any of my birds brown, the viewer will see "brown".1. Mix 3 primaries.
2. Mix warm compliments.
3. Add a manufactured neutral to another color with higher chroma. I usually use Burnt Sienna or Brown Madder. Just a little, neutralizes most colors.