Hydrogen peroxide and baking soda - make a paste and put it on the grout. Let it work for a while and scrub with an old toothbrush.
Here is one way: https://www.hometalk.com/41693881/homemade-grout-cleaner-recipe
So many different ways.
After thoroughly cleaning any grouting, it may need to be re-sealed again to prevent it from becoming discoloured and dirty again so quickly.
Hi Sue - This is the shower cleaner that we use to help keep grout and tiles clean and minimize the mildew - it works on kitchen grout too. Hope this helps! Hugs, Holly
PS - We have a FREE eBook with recipes for all of my homemade and all-natural cleaners. You can download it here --> https://pinkfortitude.com/thank#GreenClean
There are so many different ways to clean grout that it's almost a personal preference. My mom loves using toilet bowl cleaner and I use hydrogen peroxide. Just squirt it on, let it sit a minute, scrub with a toothbrush and wipe it up.