z wrote:
When replacing an old tile to match the old ones

or perhaps just whiten up the old ones ?

This stuff isn't bad:
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=102589&ts=81844&id=98439

I think if I were you I'd be concentrating on cleaning up or renewing
the old grout rather than trying to make the new stuff look old and
grubby...

David