I'm not entirely convinced the problem here is with the tiler. I feel it's a colour expectiation from a natural product that hasn't been explained particularly well.
Another common problem is whoever did the tiling display in the Fired Earth Showroom may have done so with a particularly white or clean batch of Carrara mosaic that maybe isn't representative of what is supplied.
I have 30mm Carrara Marble worktops in my kitchen, I had to search high and low to find a nice white slab that I was happy with. Much of the Carrara now being imported into the
UK is a dirty grey with the odd splash of white, rather than a crisp white with a splash of grey.