The longer you allow alkaline cleaners absorb into the floors, more it brakes up the dirt and grease. It is suggested to wait 20-30 min.
Scrub with a grout brush mind your walls, vacuum or mop up solution after the scrubbing is done! Rinse floors with warm water. Dry with cotton clothes then you can apply sealant.
After repeating this process you will need a better way and may consider color sealing your grout.
Grout is cleaned up to 85% of it’s natural color, 15% of discoloration comes possibly from bleaching or grout discoloration from the installation.