The important thing to remember with grout cleaning is unless it is epoxy grout you are trying to clean coloured sand and cement in other words mortar.
Firstly you must in all cases pre wet the grout so that it is full of water, This stops the cleaner when applied from sinking straight in.
So
1. Pre wet
2. Apply a diluted high alkaline cleaner (I reccommend Aqua Mix Heavy Duty Tile & Grout Cleaner or Aqua Seal XTreme HDT&G Cleaner)
3. Agitate it and allow it to dwell (Sit) Then agitate again.
4. Vacuum the solution up with a wet vac.
5. Rinse and repeat vaccing.
If required offer to seal it for the customer
Get money go home
Kev Martin
Marble Life Ltd