new grout is patchy in colour

[nottwins]

Just spent way too much having the main bathroom completely redone. It's finally finished but I'm not happy that the grout has quite noticeable variations in shade.

The builder tells me it's totally normal and none of his clients have ever complained before - didn't even suggest coming round to see it (done by his tiler subcontractor).

He says it can't be anything his tiler has done but on googling it seems that this problem can be caused by adding too much water to the mix, not stirring at the right time or using hard water (which ours is, but he is local so I would expect him to know this or check with me first).

I don't even know if there's anything I can do to fix this. What should I be asking for/saying? He just wants his money straight away and I'm reluctant to pay up in full until all the problems are sorted (although happy to pay the vast majority).

Help!!