Grout sealer will make the problem worse normally. A grout stain will probably work. I would tell you the stain that I would recommend but we get banned from forum for helping people. This color difference can be one of two things..maybe three. Too much water during wiping....inconsistent water mixture...different dye lots of grout. There is an excellant stain out their that will work...it can even be color matched to the grout you expect it to be...

I had a horrible problem in my bathroom with Customs Prism Sure Color Grout.....ya know, the grout I paid extra for so it would be "the best" with guaranteed color through out. NOT! Light here, dark there,(can you say speckled?) and certainly not Sure Color! We contacted the company, a rep came out, and he did ask that we give it a good three weeks to cure...sometimes that is what it takes. Oh, and he also asked us to spritz it down with water every day or something or other. It didn't help. The route we took was to "stain" the grout with a product they have for this. Customs paid for our tile dude to do this and it was a lot of work. I was very skeptical that it would solve the problem......but it did! The grout looks completely even now. Alas, I can't attest to the durability of it as this is a guest bath in a lake home, and the bathroom has not had much use. HTH pup

Wow,no doubt the workmanship is poor overall. What should be done depend's on what kind of agreement was made before job began. I've hired work done knowing quality might not be the best simply because it was costing half of industry standard. My scope of work and expectations are far different between a new $400k+ home and a $50k low rent district house. In either of those homes,I expect grout (esp floor) to fill gap so that cleaning is easier plus wear and tear can expose fresh grout before void/dirt trap become's overly deep.

Movement in the floor by the sounds of it. What size tile was there previously? If not large format tile was the calculation done to insure that your flooring system has the proper deflection and weight carrying ability for the the large format tile? What is the subfloor? Prep done to the subfloor? How flat was the subfloor? Full mortar set on the new tiles? The fact tat they skim coated existing grout (which does not ever work well) tells me all I need to know. Is this a true tile professional or someone who does tile?