I don't understand why it makes any difference. As long as you keep the grout lines clean as you go along, it shouldn't cause any issue. The only time I purposely use white adhesive is if I was using a white or very light grout, just in case of any adhesive left in the grout lines staining the grout. Otherwise if the same adhesive in grey is cheaper than white, I would use the grey...... I might be missing something obvious here though.