We use the RIT brand of tie-dye. Below are the instructions from their website on setting the colour to prevent it washing out:

  1. To help set the dye, we recommend using your microwave. Wrap the dyed fabric in plastic wrap and seal the edges. Place the wrapped fabric on a paper towel in the microwave and heat for 1 to 2 minutes. Plastic will puff up, creating a heat chamber that helps bond the dye with the fabric. Once the microwave is done, remove the fabric and open the plastic, letting it cool down.
  2. Rinse in cool water until rinse water begins to run clear.
  3. Hand wash in warm water with a mild detergent, rinse and dry.