Can you please tell me if it's possible to dye black or gray clothing & if so, how? I couldn't find the answer on your site. Thank you.

Are you asking about dyeing clothing that is already black or dark in color to a brighter or lighter color, or are you asking about what is the best way to dye clothing to make it black or grey?

Dyeing clothing that is already black or dark in color to a brighter or lighter color.

Dye is transparent. This means that if you dye anything that already has a color, the result will be the new color added on to the old one. Overdyeing black results in nothing other than black. 

In order to change black clothing to a lighter color, you must discharge, or remove, the dye that is already in it. This is not always possible. Some dyes will resist any discharge chemical, retaining their color even after the fabric itself has been damaged. 

Synthetic fibers of any sort, as well as protein fibers such as silk or wool, must never be subjected to hypochlorite, which is the active ingredient in ordinary household chlorine bleach. An alternative to bleach, Rit Color Remover, which is based on sodium dithionite (also known as sodium hydrosulfite), is less damaging to protein or synthetic fibers; it works well on some dyes, but not on other dyes. You will need to buy several boxes for one washing machine load; in the US, you can find Rit Color Remover at most fabric stores as well as many grocery stores and pharmacies.

If you use bleach to remove the color from 100% cotton garments, you should neutralize the bleach, after rinsing. Do not use vinegar for this purpose. See the instructions on the following page from my FAQ:
How can I neutralize the damaging effects of chlorine bleach?
http://www.pburch.net/dyeing/FAQ/neutralizingdischarge.shtml
You do not need to chemically neutralize after using Rit Color Remover.

After you have used chlorine bleach or Rit Color Remover to remove the dye from your fabric, if that turns out to be possible, you will probably find that the polyester thread used to stitch the garment together remains the original color. There is nothing that can be done to solve this problem, if it occurs.

Bleached fabric can be dyed like any other fabric of its color. Any remaining color will be incorporated into your final color results. You should choose your dye carefully based on the fiber content of your clothing; see "About Dyes", at http://www.pburch.net/dyeing/aboutdyes.shtml . Cool water fiber reactive dyes, such as Procion MX dye, will give better results on cotton than you will be able to obtain with all-purpose dyes, such as Rit dye.

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