From: stuart noble
Date: Fri, 10 Sep 2010 Time: 19:42:28

Not sure why the acid would have produced a brown colour

It's the action of the acid on ferrous fragments in the stone. See the
para beginning "York Stone" in:

http://www.pavingexpert.com/maintain_02.htm#acid

and the para headed "Concrete Paving" under the pictures.

--
Ian