Remove any dirt from your rubber-backed rug by washing on a full cycle in a washing machine. Wash with a mild detergent such as Synthrapol, which is recommended by Jacquard Dyes, maker of Procion MX fiber reactive dye.
Prepare your dye well in a large bucket, pot or washing machine. Mix together 3 gallons of hot tap water, salt and dye. Adjust your dye amount for desired color effect. For a lighter color effect, use 1/2 tsp. dye. For the darkest and brightest color use a full 2 tsp.
Immerse your rubber-backed rug in the dye mixture for 10 to 15 minutes, stirring frequently. If using a washing machine, alternate between agitating and soaking, though be sure not to let the cycle run through.
Remove the rubber-backed rug from the dye mixture and place on drop cloth. Stir 1/4 cup soda ash into the dye mixture until dissolved.
Place the rug into the dye mixture, and stir or agitate for 30 minutes to 1 hour depending on the desired intensity of your color.
Rinse your rug thoroughly, or allow the washing machine to complete a cycle. Follow up with another wash set to hot, using Synthrapol as detergent. Allow to air dry.