I use a paint colour, Burnt Sienna. It is a warm orange brown. But I also mix paints to create brown colours. You can mix warm colours with green and make a variety of brown colours. You can mix orange, red or yellow with green paints. I don’t like to fully mix my colours. You can achieve a more interesting colour when you under mix your paints. To make gray colours, you can mix complementary colours, i.e. orange and blue. Grays have a cool hue base. If you add a little white to your gray mixtures, you will achieve an attractive gray colour.