What advice do you have for a commercial kitchen floor where the grout has worn in areas mostly of higher traffic. I need to replace the grout in a friend's kitchen.

I know there are lots of choices of types of grout to use. We're using a "black" grout on red tile.

With the water, grease etc in a kitchen, working in their off hours what type of grout do you think will have both the greatest wear as well as ease of installation? Do I need to remove all the old grout for the best outcome? Also, with whatever you suggest, can you tell me how much time between when I'm done and when the floor can handle traffic? Thank you!!