Mike, just out of interest why do you specify white for the mosaic?
Musivarius, I don't know what type of mosaic Dan has bought, but did caveat my response if glass or stone then use white (because if stone its most likely travertine which must be white).
If they were ceramic or porcelain mosaic then grey would be fine.
I have always been led to believe that glass mosaic tiles are just clear glass with a colour coating on the back which give the appearance of colour to the mosaic. I have also been led to believe that white adhesive has the lowest risk of adversely affecting that coloured coating, for example making it look darker, and so have always used white (and never had any problems).
I may be wrong and grey may be perfectly okay, but as the adhesives that are sold specifically for fixing mosaic, e.g. BAL Mosaic Fix are also only available in white, I figure there may be some truth to the fable.
I see you are a mosaic expert, gulp. Was my advice right, or wrong....??