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 was considering quartz and went to three different showrooms. All staff insisted the counters would not yellow from direct sunlight. I've see that asserted as a fact in quartz threads but I'm not sure I recall anyone saying they had experienced it themselves. My kitchen is flooded with natural light, so I asked specifically. (But now I guess we're going with stone, since I found out DH prefers it!)

I suggested the sanding/de-glossing as a solution because that happened to me on one side of my kitchen island. My quartzite is honed and on one side the edge we used (1/4 or 3/8") looked shiny very close to the edge. It was like a little 1/4" strip next to the roundover had been inadvertently polished to high sheen by whatever does the roundover. They came out, fixed it with some kind of compound that felt like the texture of whole grain flour (but made of stone), and it made a freaking MESS but it was all better when they left. They had a machine that looked like an angle grinder but with a wierd rubbery head on it. Joseph probably knows what it is. So ... that's the reason I suggest this. It didn't remove any material that I could perceive from the counter along that edge. It was just a surface treatment to even out the sheen.

the trouble is not everybody likes granite. I hate it. all of it. too bad about yellowing resin in manmade quartz though. it's what I was planning, but my kitchen can get some harsh sun. I suppose I will be looking into other products. as far as UV window stuff: some of us don't have it in the budget to swap out our perfectly functional windows when updating a kitchen. besides, it's like sunscreen: it only cuts down on the harmful rays, but not 100%. not everybody remembers to close their blinds every day (me). I had tinted windows in my last condo (south facing), and we often closed the curtains, but not every day. the carpet and artwork in that room faded after a few years.