Based on the question, I assume the grout utilized did not have stain resistance characteristics and was not an epoxy grout.  In that case, a penetrating sealer should be used and it is like painting a tic-tac-toe pattern which is simple, but tedious labor and can easily be done by any flooring installer laborer or (DIY) patient homeowner. You will get good results from the use of a grout sealer as a typical neutral cleaner will remove any dirt, etc. from the sealed grout surface.