How to get your old dirty grout looking like new again

Go from this

to this

I have tried many things to clean the grout but it never comes clean and bright. Then I heard about this product called Poly Blend Grout renewal. It’s actually a colorant. To put it on they recommend a toothbrush and then use a slightly damp cloth to wipe off any excess as you go. You can buy it at Home Depot or on Amazon.com