On 2007-07-22 22:20:38 +0100, (A.Lee) said:

I laid a patio last week. All went well, but the owner insisted that he
finsihed the grouting between the slabs.
OK, no problem.Leave him to it.
The slabs have a yellow tinge to them, and the owner has now told me he
has grey concrete stains on them from over spill when he grouted, and
will I go round and have a look.
So, whats the best way of getting the light discolouring off of these
slabs?
Ta
Alan.

You don't say what the slabs are. Concrete, limestone, slate,.....??

If it's material that is not soluble in acid (at least quickly), then
brick acid could be a solution. It should be tried on a scrap piece
first.