Ok (bites tongue)......
Your grouting is clearly efflorescence. You've used far too much water to wash down. You can see where water has been left within the joint, particularly on the corners.
Charcoal I find is one of the most sensitive colours to this. Would of been an advantage, so something to do in the future, after your last mop run over with a soft cloth to remove any excess water. Then once dry buff to remove any remaining grout residue.
Floor tiling: 6mm is no good for any tile on the floor IMO. What tiles are you fixing?
Depending on how good and flat your surface is you could possibly use an 8mm but generally speaking a 10mm notch is used.