I'm kinda joking, but kinda not, was watching "Pitchmen" last night on Discovery Channel and Billy Mays was talking about his products, as per usual, but I have heard good things about this: http://www.kaboomkaboom.com/product.m?sku=17511
Worth a shot before the maddening task of attempting to remove grout (been there, didn't go very well, ended up just ripping down the wall).
I'll try anything before regrouting. Saw another solution on one of the other forum groups. This one is for taking the mildew out of silicone around the tub. Mix bleach and baking soda into a paste, lay it on thick, wrap with plastic wrap and sit for a day or more. Apparently it removes the mildew.
If the grout in a shower is "stained", you can be assured it's probably mold. I would bet the substrate is saturated with water and is not cleanable. If you could go without using the shower for several weeks and possibly blowing a fan at it too, you might get it dry enough where you could apply a grout colorant and fix it that way. It would not cure the basic problem or hidden mold though.
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