the color of your grout can make or break a tile installation. If you get as close to the main color of the tile as possible, it will give you a more seamless look - going darker will call more attention to the grout. Are you using an offset or subway pattern like in the photo above? If you want to call attention to the grout/installation pattern then you probably want to go darker.

All grout will get dirty, if not properly sealed and maintained. A darker color will show less dirt, but still get as dirty. In a master bathroom, depending on your lifestyle, you can probably get away with a lighter color - unless it is a bathroom that gets heavy traffic with shoes on.