You can dye fabric with food coloring, but only if the fabric meets certain requirements. Dyeing with food coloring, gelatin 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 coloring on fibers that are natural protein fibers. Wool, silk, angora, cashmere or other animal fibers are natural protein fibers. You cannot dye synthetic (except nylon) or cotton fibers with food coloring.
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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 1/4 cup vinegar and a quart of water for 1/2 hour to 1 hour before adding the coloring.
Heat the vinegar, dye and water mixture. This can be accomplished by boiling on a stovetop 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 colorfastness.