Guest bathroom shower surround, glossy white tiles, "regular" TEC grout, 2008 I think. MB and hall bathroom tub surrounds, glossy white tiles, TEC XT grout, 2011. No issues. The XT grout doesn't require sealing, but can be, using a solvent based sealer. We have moderately hard water. We wipe down the tub surrounds with a microfiber cloth after showering (to prevent water spots on the glossy tile). I clean the bathrooms pretty much weekly. I spray the tub surrounds with 7th Generation disinfecting bathroom cleaner spray. Let sit a few minutes, and wipe dry with a microfiber cloth. Tile and grout look as they did when first installed.

Can you take a pic from further back so we can see how the sheets match up (or not)? There is grout colourant that is supposed to be easy to apply, but I haven't seen pics of anyone use it on tile like that. I used it on my bathroom floor and applied it with a small paint brush. You may have to put it on a sponge and do the whole thing - I'd experiment first in a place not very visible.

no easy way to fix it. scraping it out might damage the tile. what's wrong w/the grout?? It looks perfectly fine. Frost is white w/a hint of gray. You couldn't have matched it better if you tried. stop fretting.

looks like pigment issues and or efflorescense. Id suggest an epoxy grout colorant , black grout is always a gamble. Id lean on tech support from the grout/sealer MFG to make sure the sealer is stripped first for proper bond if needed.