i have recently laid down a new tile floor .i then had a gyproc
contractor gyproc my house and they did not lay down any protection for my
brown grout.it now is covered in gyproc dust and i can't find anything
that will restore the grout back to its original colour.what product
is out there ,that will bring the grout back to it,s original brown
colour ?

I am not sure if this works well on brown grout, but 'Zep' grout
cleaner is incredible.  I used about 20 bottles of it this summer.  It
cleans the tiles themselves really well too.  We tried other stuff
that has bleach in it and it wasnt nearly as good.  It also doesnt
have the harsh smell that some other grout cleaners have.  Not to
mention it doesnt bleach your clothes.  You can find it at your local
Home Depot.  For brown grout I would try it in a hidden place first,
but I would honestly be very surprised if it did any damage to it.

Try this on a small area, out of the way.  Use Concrete color or grout
color, you can get it at home depot.  Mix some up in water and let it
dry on your grout.  When you are happy with the color, then seal the
grout with grout sealer.