The answer to tough stains on grout is that they are often caused by a buildup of dirt, grime, and mildew that gets trapped in the porous grout material. Symptoms include discolored, dingy-looking grout that can be difficult to clean. Solutions depend on the severity of the staining, but typically involve using a combination of grout cleaners, scrubbing, and sometimes re-sealing the grout to prevent future stains.
For mild stains, homeowners can often tackle the problem themselves with a grout brush and a store-bought grout cleaner. However, for more stubborn stains or discoloration that has set in over time, it's best to call in a professional cleaning service. They have access to more heavy-duty cleaning solutions and tools that can effectively lift even the toughest grout stains.