Mix red paint with green paint (start with mostly red, add green gradually)
Mix orange paint with blue paint (start with mostly orange, add blue gradually)
Mix yellow paint with purple paint (start with mostly yellow, add purple gradually)
Mix black paint into any warm base (red, orange, or yellow) to darken toward brown
Mix white paint with a brown base to lighten (add gradually)
Create warm brown by using red + yellow + a small amount of blue/black
Create cool brown by using red + blue + a small amount of yellow/black
Use complementary colors in small increments to avoid grayness
Test small batches and adjust by adding tiny amounts of the needed color until the shade matches