How to clean grout and whiten on ceramic tile floors
You can purchase a grout paint pen. Otherwise, I use bleach and a scrub brush.
I use baking soda (make a paste with a little water) and a soft bristle scrub brush.
For whitening, try a grout renew pen. They come in multiple colors and I know I've seen white. I recently re-colored my fireplace grout (mine is dark grey now, but they also had white). Check out this tutorial. https://jessicawellinginteriors.com/faux-cement-tile-fireplace/
Just tried white vinegar straight from the bottle and used a toothbrush to scrub the grout works like magic!