I'm guessing your tights are a synthetic material in which case it wouldn't lighten them at all. And even if they were a natural fiber, you wouldn't get a lighter gray. I've use RIT color remover to prep a very dark gray shirt for green dye and it turned straight up orange.
What's this for? It's a wig.
What's it for? To disguise yourself from yourself.
With something like that, it's just tough toenails. There's really nothing you can do about it besides buy a new pair.
Currently working on:
Benny Gecko: Fallout: New Vegas
Momoko Koigakubo: Ghost Stories (Dub)
Snow White -- Snow White and the Seven Dwarfs
Princess Tiana -- Princess and the Frog
Malory Archer: Archer
Kamala Khan: Ms. Marvel
Anna Cornonation Dress: Frozen
Vader Eloha/The Queen: OFF
Princess Suwa: Mascot of the city of Suwa, Japan