Most of the parts of a bra are made of nylon, which can be dyed with acid dyes. Some of the parts might not be dyeable, and if there are any cotton parts, they should be dyed with different dyes.

Some dyes for nylon are a lot better than others. The worst dyes will tend to come out when you wash them, but as long as you rinse thoroughly after dyeing, they should not come out when the bra is dry. (This is important: you don't want the dye from your bra rubbing off onto your skin or other clothing.) As long as you are willing to hand-wash each bra separately, you are free to use one of these poorer dyes. All acid dyes work best in very hot water, 185°F, but such hot water is likely be bad for the spandex in most bras. You can compromise with hot water that is not as hot as would be ideal. 

If you just want a cheerful color, and don't care particularly which shade you will get, you can try dying nylon with food coloring. One popular source for food coloring is unsweetened powdered drink mix, such as Koolaid. Some food coloring colors will wash out rather quickly, but are cheerful while they last; others will last a bit longer. Even a very pale color, if most of a dye color washes out, will be more cheerful than the current gray.

Rit all-purpose dye is good for this purpose also, though relatively expensive for the small amount of dye in a packet. It works significantly better than food coloring on nylon, though not as well as other kinds of acid dyes. All-purpose dye includes a mixture of two kinds of dyes, leveling acid dyes for nylon and wool, with direct dyes for cotton. It has the advantage of also coloring cotton and rayon, if you have a bra made of mixed fibers; acid dyes do not work on cotton, though they might stain it a bit. Rit all-purpose dye is known for fading quickly and for bleeding in the laundry, but that won't be such a big problem if you always hand-wash each bra separately in cool water. It often fails to yield a good black, but will produce cheerful pinks, lavenders, greens, and blues.

The highest quality dyes for nylon include the Washfast Acid dyes that you can buy by mail-order from PRO Chemical & Dye or Paradise Fibers, and the Lanaset dyes also sold by both PRO Chemical & Dye or Paradise Fibers. Jacquard Acid Dyes also work well on nylon. These various high-quality dyes are what you need for the most permanent colors, the greatest degree of wash-resistance, and the deepest, darkest colors. If you want to dye your bras black, I strongly recommend that you buy either Washfast Acid Dyes or Lanaset Dyes, because you're apt to get no more than gray or a dark purple if you try to use Rit or Kool-aid to dye something black. For just coloring your bras an assortment of cheerful colors, though, it's not really necessary to buy the highest quality dye, if you're willing to always separately hand-wash the bras in cool water, and to redye if too much of the color washes out.

I'm sorry to note that none of the above will be true if your bras have any polyester components. Those parts will just stay their original color. The only dye that will work on polyester is Disperse dye, which you cannot purchase locally. Disperse dye requires a lengthy immersion in the dye at a strong rolling boil for half an hour or longer, a harsh treatment which will destroy many bras. It is preferable in this case to avoid all dyes altogether, and instead use a fabric paint that works on both natural and synthetic fibers, such as Jacquard Products' Dye-Na-Flow or Dharma Trading Company's Dharma Pigment Dyes.

Fortunately, most bras, and the lace or ribbon trim on them, are usually made of nylon. It's definitely worth a test, dyeing one to see if you like the results; if you do, you can then try dyeing the others.

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