You can dye fabric with food colouring, but only if the fabric meets certain requirements. Dyeing with food colouring, gelatine or drink mixes such as Kool Aid includes a few more steps than if you were to use commercial dyes specifically formulated for use on fabric.
Use food colouring on fibres that are natural protein fibres. Wool, silk, angora, cashmere or other animal fibres are natural protein fibres. You cannot dye synthetic (except nylon) or cotton fibres with food colouring.
Setting the dye
You will need to set the dye with an extra-acidic formula. Most dyers recommend soaking the item in a solution of 59 ml (1/4 cup) vinegar and 1.14 litres (1 quart) of water for 1/2 hour to 1 hour before adding the colouring.
Heat the vinegar, dye and water mixture. This can be accomplished by boiling on a hob or in the microwave for 5 to 10 minutes, or you can leave the mixture in a plastic bag in a sunny place for a day.
Rinse and reheat
Rinse the project under cool water until the water runs clear. Reheat the project in the dye bath and rinse again to ensure that the maximum amount of dye has been absorbed.
Allow the project to dry completely. Wash the item separately in cold water for the first few washes to ensure colour fastness.