Water – proven technique for removing grout joints without chipping porcelain tiles

Try one of the many Grout Restorers or Rejuvenators. They would bring the problem area(s) back to matching the rest of the grout. There are also Grout Coloring Kits that basically paint or stain the grout. You can luck out if a color perfectly matches your grout, but usually you'll have to do the whole floor's grout lines for everything to match. Sealing your grout is the only preventative measure you can take to avoid future situations. This paints on a clear coat & thereby removes grout's porosity. Though, it does need to be re-done every 3 to 10-years...depending on traffic.

One way to proceed is to remove all the tiles and then install new ones using a better quality installation technique to get uniform gaps between the tiles.

This approach also allows for:

1. The possibility to update the tile backer to the most modern methods to ensure no leaks and inspect for old mold in the walls.
2. Selecting tile that may be more pleasing or modern.
3. With the walls open you have the option to replace any plumbing fixtures that protrude through the tile.