You can achieve the colour brown for food colouring in many different ways. One technique is to blend existing food colourings together. Another technique is a more natural process. Either way, the result will be similar.
Mix equal parts red and green food colouring.
Vary the amount of each colour to achieve the desired shade.
Add additional colour for more specific shades. For instance, adding a touch of orange and green will create a tan-like brown, while a warm, rich brown can be achieved by adding a touch of yellow. If the colour looks too grey or black, add more red or yellow.
Put 1 cup of instant coffee in a glass or enamel pan and add 1 cup of water.
Place the mixture over medium-high heat and bring to a boil.
Turn the heat to medium and simmer for 30 minutes.
Remove it from the heat. Cool and strain the mixture wiht a very fine-mesh strainer or coffee filter. Make sure no grounds fall through.
Follow the same method for making brown food colouring from coffee, but use 1/2 cup of cocoa or 10 teabags instead.
Omit the straining step; there are no grounds to strain out.
Understand that the colour or shade of brown will vary.