Using Earth Watercolors
The earth watercolors are already a mix of the three primary colors, however each leans a little towards one primary or secondary color. For instance, Burnt Sienna has a strong orange leaning, Raw Umber has a slight greenish tinge, Raw Sienna is a yellowish brown with a slight greenish tinge, while Yellow Ochre is obviously a Yellow with a slight red leaning.
Looking at these earth watercolors this way explains why French Ultramarine (primarily a blue) when mixed with Burnt Sienna ( Orange = Red(R) plus Yellow(Y)) gives you a beautiful dark color, almost a black at times, depending on the relative amount of each pigment mixed and the quantity of water used.
French Ultramarine + Burnt Sienna = (B + r) +(R + Y) = B+Y+R = Strong dark color.
French Ultramarine + Raw Umber = (B + r) +(Y + b) = B+Y+r = dull green color.
Full list of colors in my palette along with their color bias (leaning)
Here are the rest of the water colors in my Winsor and Newton palette for reference:
French Ultramarine: Warm blue = B + r
Cobalt Blue: Almost a pure pigment not warm nor cool = B
Cerulean Blue: Cool blue = B+ y
Alizarin Crimson: Cool red = R + b
Cadmium Red: Warm red = R + y
Cadmium Orange: Warm orange with a lot of yellow and some red = Y + R
Cadmium Yellow Pale: Warm yellow color with a little red = Y + r
Aureolin: Cool yellow = Y + b
Cobalt Turquoise: acts like a cool greenish blue, has lot yellow in it = B + Y
The earth colors are more complicated as they already have some of each of the three primaries in them. Effectively they are already greys (colors made when you mix three primaries together) which lean towards one or more of the primaries. The indications after the equals sign ( = ) refer to the colors the brown leans towards.
Burnt Sienna: Orange brown earth color, can be treated as a dull orange = R + Y
Raw Umber: Slightly greenish brown earth color = b + y
Yellow Ochre: Warm Yellow earth color = Y + r
Hopefully this information about earth pigments and their component colors will help you to become more confident with your own color mixtures.