Brown is a secondary/tertiary colour, and you can mix it using primary colours or other combinations depending on what paints/pencils you have.

Take equal parts of red, blue, and yellow.
Adjust shades:
More red → warm brown (like brick).
More blue → cool/dark brown.
More yellow → lighter/tan brown.
Each primary colour mixed with its opposite makes brownish tones:
Red + Green → brown
Blue + Orange → brown
Yellow + Purple → brown
(Complementary = opposite on colour wheel)
Add white → light brown / beige.
Add black → dark brown / chocolate.
Add more yellow → golden/tan.
If you want chocolate brown: mix red + green and add a tiny bit of black.
If you want skin-tone brown: mix red + yellow + a little blue, then add white.