The medium gray is a solid choice for sure, but I'll push back a little on the idea that everything is designed to hide mess. White grout shows dirt fast because it's a high contrast color, but a lot of people pick it because it matches white tile and makes the floor look bigger in small bathrooms. The real trick is using an epoxy grout instead of the regular cement stuff. It doesn't soak up spills or get stained as easily, so even white stays cleaner way longer than a month. Have you tried one of those stain-resistant sealers yet?
That "hides everything" part is the real win. @miles_robinson20 is onto something when he says everything is designed to hide mess, but picking the right shade of gray is where the rubber meets the road. Medium gray works because it matches the natural dirt color, plus it doesn't show water spots the way darker shades do.