If you have light color grout, you can mix hot water and alittle bleach to spray on grout lines, sit a minute before using a toothbrush.
Note: I know not everyone likes using bleach, But it works for me.
I like bar keepers friend and bleach.
Hi Judy. I'm Cindy. I recommend a product called Oxi Clean Max Force. Do a test sample first. Then use an old toothbrush to scrub it into the grout lines. Let it set for 15 or 20 minutes. Then clean it all up with a clean damp cloth. Good luck Judy.