Play-Doh is a sculpture medium usually thought of as for children. It comes in several colours. The basic set comes in red, blue, yellow and green. So what do you do if your child is a little more particular and wants other colours to work with? Use the same method you would use to mix paint colours. You can make any colour with the three primary colours.
Mix 2 parts red with 1 part yellow. Knead until you have a smooth orange colour.
Pinch off a very small amount of blue and knead it into the orange until the colour is smooth.
Continue adding blue until you have the colour brown you desire.
Start with small amounts of each colour and experiment to get the brown you like best. Red and green also make a nice brown that you can adjust by changing the amount of each colour used.