Discoloured Indian Sandstone

Hi, I have a patio with 24"x24" sandstone tile quarried from India. When installing the tile there was some concrete grout left on the tile, and to make a long story short Muriatic Acid was used to remove the concrete which worked great for removing the concrete but discoloured the tile badly (rusty colour). What steps should be taken to stop the discolouration, and is there anything that will restore the original colour of the stone?

 

Your help would be greatly appreciated

Thanks

Doug B