Well, I'll throw out another wild one. I'm looking at those joints and they look wider than 1/8". And you said he used unsanded grout? And it appears this guy like to water his grout...I'm almost thinking that if he did a lousy job the first time around, he had problems with the unsanded shrinking during the curing. Especially if he likes water. Maybe a little flaking, a little cracking, a few pinholes opening up. Then he came back on the subsequent visit and grouted again over the existing grout to hide and fill in the blemishes. But that's just too crazy for me to speculate like that. I'm almost tempted to recommend a few ideas, mainly try a Q-tip dipped in bleach to see if the dark stuff low on the wall isn't mildew, and try a bit of an acid, even mild vinegar, on the white stuff down low on the wall to see if the white stuff is efflorescence that the acid will dissolve away. I keep coming back to your first photo: In the vertical wall-wall corner joint that is grouted, the right part of the grout joint looks like the original quartz color. the white stripe on the left part of the grout joint, as well as the white in the horizontal grout lines above the first course of tile, looks like efflorescence. The dark along the tub looks like mildew or moisture. But if the light and dark have been there forever, both efflorescence and mildew take a while to form. If your water has heavy minerals or it's heavily chlorinated, that might be a player. If is he used old bags of grout that were past their shelf life that could be an issue too. Or a contaminated mixing bucket. Or... But as you can tell, I'm just scratching my head and guessing. Does the white grout have a different texture? If you scrape the white with a fingernail, does it powder off? If it does come off, is there any of the true "quartz color" underneath it? Have you tried cleaning this grout at all? Like in my first post, I just keep leaning towards the idea that too much water was used. When I first looked at the photos of the upper walls, they looked like areas of good grout with a few blotchy dark streaks. Now I look at them and see the uniform light color being almost all efflorescence, or I'm seeing grout that has had the surface cream/pigment washed away, and the "dark blotchy areas" are actually the proper quartz color! I'd love to see this shower with my own eyes to figure it out. I probably have to take my son down to William and Mary for a college visit later this month. Bill, how about a road trip? A few crab cakes, catch a lacrosse game while we're down there, we can check out this shower all while taking my son for a college visit? Too bad the Orioles are going to be at Fenway instead of the other way around. Road trip!!! Seriously though, have you tried cleaning the grout with anything? A neutral pH cleaner and a bit of elbow...

I used Aquamix Grout Colorant on my kitchen floor which is an epoxy product that seals the tile too. I did not apply extra sealer because it really isn't necessary. P.S. It's not that tedious a process. In fact, it's very easy to do and has very satisfying results.

The oddest indicator to me is your first photo, the sliver of "whiteness" on the floor along the wall in the left part of the photo. If that is truly there and not a photo anomaly, I'd not think the problem originates with the grout itself. I'd venture it has to do with something done post-installation; the cleaning, or the sealing. So many variables, but the overall blotchniness? Could be the application and use of the product used to clean the grout shmears off of the stone. If the texture of the grout was changed during the cleaning process, the sand papery grout can hold shower residue; soap and shampoo product, etc. Same with sealer use. If the sealer pooled in some areas versus others, or if the sealer dried on the surface, it can become tacky on a micro-level. You may not feel it, but it can hold dirt off the bottom of your feet, or hold shower residue. The following two things will typically whiten grout versus causing it to go darker: Could be efflorescence, but with the linear drain I'm guessing you have a surface applied membrane under the tile. Efflorescence is more common when tiling over an improperly built mud bed or a slab. Could be latex leaching. Latex leaching can be effected by water usage during the installation, as well as an insufficient grout drying/curing time before the shower was used. Latex leaching will typically lighten a surface, but the latex can hold dirt. No solid answers from me. Just speculation.

If you don't have a nice gap under your bathroom door, so make up air can enter as your fan is running, you wont move the air out the ducting. Dry the caulking and corners with a towel daily or after every shower. Don't leave the niches damp either. I have never had an issue in the 6 years I've had my new shower. I don't squeegee, I wipe with a giant microfiber towel daily. I pay particular attention to the corners and change of planes. I keep an eagle eye on the caulking to be sure nothing is growing. Nothing is growing. Grout is not supposed to be sealed, so that it can evaporate the moisture. At least that is how I understand the function of cement type grouts. Its the wall system that keeps the water out of the stud space. Same with a tiled floor, I believe. A question for the tile experts. Like others have said, good venting/functioning exhaust fan, and daily thorough wipe downs keep everything looking nice.