People use basic colour theory for mixing colours. The three primary colours of red, yellow and blue combine and make secondary colours. The secondary colours are purple, green and orange. If a primary colour and secondary colour or two secondary colours combine, a tertiary colour is created. Tertiary colours are muted, muddy colours like brown and grey. Brown is a tertiary colour created using several different formulas.
Place your paints on a flat surface, and check the colours. Make sure the red is a true red. Some reds look bluish, and some have an orange tint to the colour. These reds don't mix well. You want a cadmium deep red or alizarin crimson. Either one works well for mixing.
Create brown using all primary colours by mixing equal amounts of red, yellow and blue paint. Blend the paint thoroughly. If it looks too purple, add a little yellow. If it looks too green, add a little red. If it looks too orange, add a little blue. It may take a few minutes before the colour looks right.
Make brown by mixing equal amounts of orange and blue. This makes a greyish-brown colour that works well when painting old wood, trees or skin. If it looks grey instead of brown, add a little more orange to the mixture.
Blend equal amounts of red and green together for a rich, warm brown. It makes a cherry wood colour.
Put equal amounts of yellow and purple together, and blend thoroughly. This mixture makes a brown that works well for painting swampy areas.
Test the colour by painting a dab of paint on paper. Let it dry before using the paint. Some types of paint get darker when they dry.
Each method of mixing brown creates a different type of brown, but the combinations create multiple shades of brown. Take out a piece of paper and test the different shades by adding a dot of colour at a time. Try starting with one dot of red and one dot of green. Mix it together and put a dot of the colour on the paper. Add another dot of green to the mixture, stir and put a dot of colour next to the first brown. Continue mixing until it turns green. Try the same test, but add drops of red instead of green. You end up with at least three different types of brown. Repeat the test using blue and orange or purple and yellow.