Will be interesting to find out what they say, i just brought the same grout myself in white... i used the gunmetal grey one last week on my floor which was tiny little hexigon mosaics so a LOT of grout was used and would be impossible to remove (see photo - colour dark as its still wet, got much lighter once dry).
Might have to test it at area that goes under the bath, jon did u just rub a wet finger to see if it flakes off.
I must admit, i did weight the water and powder using good old kitchen scales, but im a diyer who hadnt grouted a floor or mosaics before so was happy to follow instruction .