I am sorry that you are having to deal with this problem. Have you or your builder contacted the grout manufacturer? Did the installer mix something with the grout that he should not have?

Was the mud/adhesive on the tiles completely dry before they grouted?They should have waited a day or two before grouting to make sure that the floors were dry. Moisture coming up from the mud, could have made the grout turn white.

The only easy fix I can think of that you may try is having the floor installer stain the grout so that you have a consistent dark color. I would make sure that the builder agrees to warranty any "fix" in case you have an ongoing problem with the grout. Someone... somewhere... during the manufacture or installation made a mistake. I hope that your builder works with his tile installer to get a permanent correction to this issue. Do not let your GC push you into accepting this.

(efflorescence which is the calcium from the water and the underlying concrete or mortar migrating up through the porous cementitious grout.) ..too soon for that. They have just started grouting.

And yes, you can grout the following day. moisture coming from the mud will not turn the grout white in one day. It's dilution. Or known as Washout. The grout guy used too much water during the wiping phase.

Some other causes

The mixing water and/or the wash water used to clean had a high content of iron or other minerals.

Water was added to the grout powder instead of the powder being added to water

Too much water in the grout will cause pigment “washout,” performance issues and a lighter- than-desired color. Too little water can lead to performance issues like powdery grout and shading (variable shades in the grout color).

Grout was not allowed to slake (sit) after initial mixing.

Grout joints were wiped excessively during the initial cleaning process.

Too much water was used during the Use a slightly moist sponge where water does not drip out. Follow cleaning process.

and no to having them stain it! They are supposed to do it right the first time. They shouldn't have to 'fix' anything. They should learn how to properly grout w/dark colored grout. If they can't handle it, then bring in another company who can. It's your money, you get to have what you want.

sorry you GC is feeling rushed. when they said they didn't want to use the dark grout, that was them telling you they aren't comfortable using it. Too bad. Too bad the GC didn't have his guys watch videos of how to grout using dark colors. Too bad they don't follow directions from the grout manufacturer. Perhaps you should remind your GC of this?

I did my floors w/black and charcoal grout and didn't have any issues. I also told my tile guys to use very, very little water during the wiping phase!

Thanks for the quick responses! I suspected that they used too much water and this was the end result. This work is being done as part of an insurance claim- we had a house fire and have been out of the home since March. Its been delay and delay and I suspect that my insurance adjuster want crazy on my GC, as they have been missing multiple deadlines. This has lead to the crazy push this week... Unfortunately, it feels like we are not fully in control, but this is work is unacceptable.Given the timing push, things have felt very rushed and I don't think everything is being handled properly- clearly in looking at the end result it is the case.

They used Mapei Keracolor Unsanded Grout in black- I went with black because I had seen some dark brown grouts that almost showed up as mauve and want to ensure a strong color. I also thought it somewhat mimicked hardwood, as the wood I had in my previous home was always darkened at the joints.

What should I push them to do as next steps? Its sounds like having them remove and start again rather than stain? If they do stain, can that be done immediately or does it need to wait? I feel like they are really going to try to pushback on this since they are being pushed on, so I just want to make sure I fully understand my options.

I wouldn't stain at this point. it's not going to match the rest. (unless they're going to stain everything which is just silly) have them remove what they've done.

How wide are your grout lines? less than 1/8th? if not, they should use sanded grout.

if you can, scrape it out with a tool (take care not to scratch the tile). you only need to remove 2/3rds of it for them to redo.

After reading some of the tech. data on their site, I found this:

Due to the increased importance in color quality and consistency and the challenges referenced above, MAPEI recommends using MAPEI FlexcolorTM CQ,Kerapoxy® CQ and Ultracolor® Plus FA with wood-plank tiles.

http://www.mapei.com/public/US/linedocument/TB_Installing_Wood-Look_Porcelain-EN.pdf

all of the browns looked mauve? what about Chocolate or Cocoa? Maybe try the UltraColor Plus. Looks like Keracolor is limited on colors

http://www.mapei.com/US-EN/coloured-grouts-list.asp

I used Mapei Flexcolor CQ in the black and the charcoal. It comes premixed. The issue w/that kind of grout (it's a urethane base grout) is the clean up. I had to tell my guys to watch the video (in spanish) of how to properly clean it and how to use very little water. On one portion he didn't listen and I ended up having to clean all the residue. took me hours to do.

the grout you're using is a cement base. darker colors are going to get 'washed' if they use too much water during wiping.

http://www.mapei.com/public/US/linedocument/TB-Causes_and_cures_of_efflorescence-EN.pdf

just tell the GC the white is unacceptable, and he better advise his guys how to properly grout or else they'll be redoing it. Watch the video yourself so you know.

sorry you're having to go through this mess. 9-10 months out of the house is a long time. I'm sure the GC just wants to be done. Not an excuse to get sloppy.

I just had this issue in my new home.
I had selected a darker brown grout and 2 days after install, it was white. The cleaners tried vinegar and then an “acid wash” to remove the white. It helped a little, but the grout was still way too light. The owner of the tile company insisted the white was just “dust from the construction.”
I knew better. He even told me to stay off the internet after I mentioned maybe too much water was used during the install.

After a ton of research I ended up applying Aqua Mix enrich and seal. Absolutely love the result.

See the before and after in my pics where blue is before and red is after.

If you put water on your white grout areas, does it turn to the black color you like? If so, check out the enrich and seal (especially if regrouting or staining aren’t options).