I
Italy
exactly, these laws are1) The tiles were bought from a 'reputable' source after receiving a sample. One would expect that sample to be representative of the stock held now, not 3/6/9 months ago! However this is hardly ever the case and they do advise to check shades before fixing.
I would check the batch/shade / quantity of tiles before I started any job.
2) It is not acceptable in my opinion to fix tiles that have been supplied - because that's what he's been given! As a professional ( and he didn't have a wet cutter!) it's not reasonable to assume that the tiles have been checked by others.
3) Once the shade variation had become apparent he should have mixed the pieces of mosaic - piece by piece if required, to get an acceptable blend.
Saying all that I'am not sure if he could have appeased a customer at that stage.