"A quick fact about the grout used when your tile was installed. Grout is made with a water repellency built in to provide years of use when maintained. In a perfect world, your grout should look shiny and bright forever. In most houses, even with the best use of cleaning practices, yellowing and mildewing still persist. So why does this happen?
1. Consider changing your shampoo
When you read the ingredients listed on the back of your shampoo bottle, do you see dyes and colorings in the product? If so, choose a new shampoo that is more natural without colorings in them.
You will be doing your shower a favor, and more importantly, your body. Dyes are not good for you or your family's health and can also stain the grout in your bathroom.
2. Stay away from harsh cleaning chemicals
Harsh cleaners may make your grout look cleaner, but they also strip away the water repellency that we talked about earlier. Bleach is especially bad about this.
A mixture of approximately one-third white vinegar to two-thirds water is what I recommend to my customers to clean grout. Use a soft scrub brush with a little elbow grease."
Read the full article at: https://www.angieslist.com/articles/keep-your-shower-tile-grout-looking-new.htm