Mix red and green (equal parts for dark brown; adjust with more red for warmer brown or more green for cooler brown)
Mix red and blue to make purple, then add yellow to shift toward brown (add small amounts of yellow until brown appears)
Mix orange and blue (start with orange, add blue a little at a time to reach brown)
Mix yellow, red, and a small amount of blue (start with equal yellow and red, then add a tiny bit of blue)
Use burnt umber as a base, then adjust with burnt sienna (more burnt sienna for warmer brown, more burnt umber for cooler/darker brown)
Use raw umber as a base, then add tiny amounts of black or ultramarine (black for very dark brown; ultramarine for cooler dark brown)
Warm brown: add a little more red or burnt sienna to any brown mix
Cool brown: add a little more blue or a touch of green to any brown mix
Lighter brown: add white gradually
Darker brown: add a small amount of burnt umber, ultramarine, or black gradually
Test on scrap first, then fine-tune by adding one color at a time in small amounts