How can I dye this fabric?

Sorry, but No, latex rubber elastic can not be dyed.

Do you still have the fabric/care label or do you know what the material content of the bras is? Elastic used in lingerie is not always latex. It can also be polyester, nylon, lycra, or a combination, all which are synthetics and can not be dyed.

Also, as far as the body of the bra, it is most likely a combination of various synthetic fibres. 100% cotton is rarely seen nowadays because it shrinks and does not stretch.

My suggestion is to soak the bras in hot water and scrub the stained areas with shampoo. Shampoo is designed to wash out scalp oils, and is sometimes effective in removing body sweat from clothing.

I have painted my bras with acrylic paint watered down. I gave it a Monet look using about 6 different blue and greens and a few spots of yellow gold. I belly dance and if O really like a bra I stretch over a box bigger than I am and sew velvet over the whole bra by hand and then add shells, beads and sequins. I dance for just the fun of it so I create my costumes so the look like the outfit that Princess Leah wore or something else. Have fun, little watered down paint a box some beads and some razzle dazzle later you have a bra you could wear in public.

Try Rite dye

Go to a fabric store and ask about fabric paints. There are paints for all types of fabric.

Rite dye should be good!