What type of construction, tract or developer home? Custom? Semi-custom? The type of construction often gives clues as to what the issues may be. None of this sounds good however and the fix will be dependent on the symptoms at the end of the day. And all changes in plane should always be 100% silicone caulk, never grout.

I have exactly the same issue. The tile guys working on my house were generally incompetent, so I assume they probably didn't follow grout instructions or mixed it incorrectly. Also, I purchased for them Mapei Ultracolor Plus FA, but they seemed to use it together with a regular cheapo generic grout. That may also be the reason. The color is off in places, and it gets dark when when wet.

There may been some settlement of the house and the grout cracked. Many recommend caulk vs grout for ”changes of plane” In you shower, floor to wall and wall to wall.

Many new houses ”settle” after being built. If the area was waterproofed before tile installation, you have no worries other than cosmetic.

The tile industry has a ”book” TINA

It is a 1 year old custom home.

I don't understand why I'm seeing white showing thru from under the gray grout? Is that thinset? Did they push the tile too deep into the thinset? And then only put the thinnest of veneer of gray colored grout to cover the white thinset? Which then flakes up?

I also don't understand why in some spots when the grout gets wet - it stays dark/wet for 24 hours? But in other places it drys and returns to light gray within say an hour or so? It suggests 2 different types of grout? Or 2 different levels or rates or degrees of absorbing water that take 2 radically different time-frames to dry out?

The 2nd bathroom shower does not have any of these issues. Just one shower.

I don't understand changes in planes? Going back to my geometry days... There's 4 walls which are in 4 vertical planes at 90 degrees to each other and the floor. There is a floor with is on a horizontal plane. I count 8 changes in planes ignoring the ceiling which is not tiled. Two of those have issues. But the majority of the issues occur in varying random spots around both the floor and the walls and not where there are changes in planes. E.g. only a very very small minority of the total linear inches of problem grout occurs at these changes in planes. Some I'm confused as to why that would be the focal point of the 2 issues?

Thanks for any clarification

A change in plane is every corner, in this instance inside corners, the reasoning is different walls will expand and contract (move) ay different rates and the grout will crack and fall out. Also silicone should be used at transitions for the same reason. The grout cracking in those locales is not a matter of if, only a matter of when. It always happens.

OK.

What about some parts of the grout (not in changes of plane) staying moist (and therefore much darker) for 24 hours and other grout (again not located in changes of plane) drying in an hour or so?

Also what about the thin-ish gray grout (veneer) flaking off (not in changes of plane) and revealing 'white grout' under? Could the white be thin-set where the tile was pushed down too far into it? Is there a depth minimum that should be left to allow the gray grout to 'bite' and hold?

" What about some parts of the grout...................."

Holding water. Possibly not drainging properly. How was the shower constructed? Was there a flood test before the floor was tiles?

" Could the white be thin-set where the tile was pushed down too far into it? "

Quite possibly.

"Also what about the thin-ish gray grout................."

Typical should be 1/2 to 2/3 the tile thickness if I am not mistaken.

Hee's a YT channel on grout. Link. Seeing white under the gray is likely the thinset used to set the tile squeezing out when the installer pressed the tile sheet into the notched thinset.

As others have said, the white is thinset. While I'm an amateur tile layer, over the years, after five bathrooms, two laundry rooms and a foyer I've gotten pretty good at reducing the amount of thinset that oozes up when the tile is worked into the thinset while still maintaining almost 100% coverage under the tile when I pull up a random tile to check coverage from time to time. It's not that hard, just needs to be paid attention to.

So I've confirmed the white is thinset. I guess they can scrape it out or enough of it out to allow sufficient depth to re-apply the gray grout and cover it up. It's a cosmetic issue.

On the wet spots on the bottom I've learned a fair bit more and could use some further feedback from the community...

The wet spots basically stay darkened and seep water for 3-5 days after usage. (we started using the other shower for now). After a couple days - I applied a heat gun and got the wet spots to go away (temporarily). Checked an hour later and it was dark again and ever so slightly damp. But after 3-5 days - the spots get dry, lighten in color and stay dry. That is until the shower is used again. And so on. There are no water lines in that wall either.

My theory is that water is getting behind the grout up above and somehow running down thru whatever channels it can find and pooling lower where it eventually seeps back out over time... Purely speculation on my part.

I found one particular gap about 2 or 3 inches wide a couple courses directly above the wet spots occur and I can actually see blue backer board where the grout is missing. Not only could water get in there but if there's not any visible thinset - that would indicate cavities behind the grout... If this is the same in other areas - there could indeed be channels the water might run down to the wet spots below...

I also see about 20 inches or so in total where there's some cracking and the tile is not making contact with the grout - I'd call it 'shrinkage cracking' except grout doesn't or shouldn't shrink as far as I know. And there's one area where the grout doesn't seem particularly hard. When dry it's crumbly to the finger touch and when wet it's 'pasty' to the finger touch. Very strange.

Mapei grout.

I think I'm going to request they remove/re-grout suspect areas above. And I think they'll be willing to seal it too (don't know how long sealer works). And I think I'll ask that they leave an inch or so of the worst offending wet areas below alone. That way if we fix the problem above, I can see if the area below drys out normally or stays wet. If it dry normally that woudl suggest the problem was fixed above and I can come back and touch up that last bit myself later.

Anybody have a better idea or input? Thanks!

" guess they can scrape it out or enough of it out to allow sufficient depth to re-apply the gray grout "

All changes in plane are to be silicone caulk, never grout for that exact reason.

" My theory is that water is getting behind the grout up above and somehow running down "

Your theory is incorrect, grout is not nor was ever intended to be waterproof. Water gets thru there constantly. You issue with the standing water is the pan is not pitched properly most likely and that will never change without tear out. The floor was not built properly BEFORE the tile was installed.

" and I can actually see blue backer board where the grout is missing. "

What was the "blue backer board" As asked earlier, what was the waterproofing system used?

" I also see about 20 inches or so in total where there's some cracking and the tile is not making contact with the grout - I'd call it 'shrinkage cracking' "

Your shower is not constructed properly and you are not "fixing" this. It will only continue to get worse and worse.