Woolite Dark contains dyes.
There, I said it.
Oh, I don't know at all, and as their formulation is a trade secret, I'll never know. But it must contain some sort of colorant, or it simply doesn't contain oxidizers, so it doesn't make clothes more faded, which looks like an improvement. Or it does nothing. They could have added mordants, which essentially re-dye your clothes with leached out dyes. Except that guess of mine is crap -- a dilute, improperly formulated dying bath isn't going to dye anything. Unless you see white trim on dark clthes becoming colored when washed.
But with the many different dyes, and many different fabrics, no one's going to write a balanced chemical equation for dye-less color recovery, or track down the rare "adds color" element.