I used Procion MX dyes and did a 5 step gradation between each color. The blue/orange combination yielded some of the best browns.
The browns from the yellow/violet combination were ok, although I didn't find them as rich as the blue/orange browns. I normally use a warm yellow to make browns with violet and they come out quite different than the colors below.
The red really dominates the blends of red/green. In order to get a nice deep earthy green, it's obvious from this experiment that you wouldn't need much red!
My next step will be to do the same blending exercises with the secondary and tertiary colors on the color wheel. I may also do some more experiments with the primaries using a larger gradation. I'll post photos when I get that completed.