I also noticed there are some companies offering "grout colouring" service, which I assume could be a solution for this -- at my builder's cost of course..
Last thing we heard from supervisor is, he is going to arrange for the cleaner to somehow scrape the top thin layer from the grout and then apply some coating on top to see if that fix it. As a "proper" handling of work like this would take ages and the chance of unnecessary damages to the titles is quite high, we are a bit reluctant to go this path but looks like there's no other option available.
Grout colorant from Aquamix is an excellent product. As long as the grout is in good physical condition, it is a good remedy to this situation. It is a one part epoxy, and very durable. For best application, contact Aquamix for a trained applicator who can do it for you.
The scraping idea sounds wrought with problems! Doesn't sound a good idea, more like a band-aid remedy that may not last long. I would NOT accept it!
Not sure, but A'nge might even be able to inspect it for you, and give you a concrete verdict of what the best remedy is (for a fee of course). I'm sure he will be happy to look at pics by email, and give some advice though.