It is virtually impossible to tint or dye a top or pair of shorts constructed from 100 percent spandex bescause of the materials used in manufacturing and its tight weave. However, because the majority of cotton/spandex blends are made from a majority of cotton -- which is a porous material that easily accepts dye -- it's possible to change the color of your drab or stained cotton/spandex top or pants. Wash your cotton/spandex top or pants before attempting to dye to ensure the material accepts the tint and you achieve the color you're after.
Fill a clean, 5-gallon bucket with 3 gallons of hot water, 1 cup table salt and 1 tablespoon liquid laundry soap. The table salt and laundry soap both help the garment absorb the dye more effectively. Ideally, the water should reach a temperature of around 105 degrees Fahrenheit. If you cannot reach this temperature with water from your tap, supplement the cooler water with water warmed on the stove top or microwave. Either way, pay attention so that you don't add more than 3 gallons of water to the bucket.
Add the powdered or liquid dye to the plastic bucket. The amount of dye required to tint your cotton/ spandex piece depends on the garment's weight and the intensity of color you wish to achieve. Follow the directions on the package, but in general add approximately 2 tablespoons liquid dye to achieve a deeper hue and 1 tablespoon for a subtler color. Stir the water with a gloved hand or large wooden spoon to incorporate the dye.
Submerge the cotton/spandex garment into the dye bath, paying attention that very inch of the piece is underwater. Agitate the water every 5 to 10 minutes to ensure the garment's surface is dyed evenly.
Lift the garment from the dye bath after 30 to 45 minutes. If you're not satisfied with the color, submerge the garment back in the dye bath and check it again in another 15 minutes. However, if you've achieved the desired color, carefully lift the cotton/spandex garment from the dye bath, wring out any excess liquid and place the piece inside the washing machine.
Wash the garment on a warm cycle without any detergent or fabric softener. This helps remove any remaining unwanted dye from the fabric. Once the final cycle is complete, dry the garment according to its label's directions.