Pants left damp for period of time turned off color, now what?!

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I have a pair of yoga pants (not sure if the material exactly, but once I look at the tag I'll be glad to share what material it is), and the pants were accidentally left damp and now they have turned an off color- almost like bleach got on to them. Any suggestions/tips/tricks of what I should do/can do? If so, please leave them below it would be greatly appreciated! I can later post a photo of the stain on the pants too. Google suggested me using fabric dye after properly cleansing the pants, to dye them back fully black. I've also never have had too much success & mixed reviews using the liquid Rit brand dye. I've never used the black one before, only a couple of fashion colors. I tried adding vinegar & salt to the mix and neither have boosted the color payout etc.