Help with dying / removing overall stain protection

[Northender]

 
Ok, so I have now hit that low point in my build. Everything seems to be going slow, nothing right and one step forward, two steps back. 

I have a set of black overalls that I wanted to lighten to a grey, not by a great deal, but still grey. I washed, bunged in the dryer with softener sheets (as mentioned somewhere). I then hit with some predye, which seemed to lighten a little, making me think it was working. I then washed with a grey dye, and voila, back to exactly the same shade of absolute black they started. 

Does anybody have any ideas on how to remove the stain protection as I imagine this is what is causing the problem? Or am I completely going the wrong way with what the problem is?

Also, I remember somebody posted a link to a person that make vests with an elasticated back to make them adjust and give a fitted look. Thought I had bookmarked, but apparently not. It had a few good pictures in it, and considering I am now about to start making my fourth iteration of my vest, I want this to be the last. (for this build anyway).
Thanks

[Kora]

I would imagine if you're trying to turn black into grey all you want to be doing is stripping color, not adding it back in. (Unless you do something that strips too much.) Have you tried using a dye remover? Both iDye and Rit brands have some (I'm sure there are probably others), but I've not used them so I don't know how much color they'll actually be able to strip out of the coveralls. (And if they do work, and work too well, you can probably add some grey dye back at that point without it turning back into black.)

[Niabi]

Yer im going to agree with Kora, strip the dye before adding, you could always try bleach? it might work? or might turn it a funny colour. 

[Nickel]

You definitely want to use a dye remover or a diluted bleach solution before you try to dye the fabric.

If all else fails, you can always sun-bleach it.  Lay it out flat in a sunny area and wait a week. Then flip it and repeat.  Adjust your time as needed to achieve the color you want..