Dyeing a shirt gray may seem like a tricky process, because you need to use black dye in a certain way to get gray. It is actually a fairly simple process to dye a fabric, such as a plain white cotton T-shirt to a shade of gray. You essentially need to just use less black dye depending on what shade you want. Follow a few steps to dye your fabric in just the right shade of gray you're looking for.
Weigh your shirt on a kitchen or postal scale.
Machine wash the shirt on the "hot" setting, which serves to scour the fabric.
Put on the rubber gloves and an apron or old clothes.
Use 1 tsp. of black dye powder for each pound of fabric for a pale shade, 3 tsp. for a medium shade, 6 tsp. for a dark shade and 12 tsp. for dark black. Dissolve the dye in two cups of room temperature water per pound of fabric and set aside.
Prepare the dye bath in the bucket, adding 2 1/2 gallons of room temperature water per pound, along with 1 lb. of salt for light gray, 1 1/2 lbs. of salt for medium and 2 lbs. for dark. Add the dissolved dye and stir some more.
Add the fabric and stir continuously for 10 to 15 minutes.
Dissolve 5 tbsp. of dye activator for light and medium gray fabric, and 7 tbsp. for dark gray, in two cups of warm water.
Remove the fabric from the bucket and pour in the dye activator. Stir it a little and put the fabric back in the bath. Stir continuously for five minutes and then stir every five minutes for the next hour.
Remove the fabric and rinse thoroughly in a bucket of room temperature water, changing the water three or four times.
Wash the fabric in hot water along with 1/2 tsp. of synthrapol per pound of fabric. Do this twice if you are going for a dark gray. Rinse with water and repeat the process until the rinse water is clear.