The choice of grout color is a matter of design intent, practicality, and personal preference. There are no right or wrong answers to this question, but there are a few things to consider.
Pattern or Texture
Once you have the tile color and pattern selected the next and last selection is the grout color. The first question I ask is “Is this installation about the color and texture of the tile?” or “Is this installation about the pattern of the tile?”
If the installation is about the color and texture of the tile, I would consider picking a grout color that is non-contrasting with the color of the tile. The grout will blend in with tile, the grout disappears and you are left with a wall of color.
If you would like the design of the project to be about the pattern of the tile and you want to show off the shape, a contrasting grout is the best choice. Dark grout and a light tile make the pattern of the tile pop.
Usage
The next item I consider is where in the home the installation is. If the project is a mud room floor, for instance, I would shy away from lighter colors as they are likely to stain and create an ongoing maintenance headache. For the walls of the upstairs guest bathroom that won’t see much use a bright white would be fine.
As a personal rule I figure that the lightest and darkest grouts are the most difficult to keep clean and dust free. Jet black grout always gets a bit lighter with dust. The bright whites often darken slightly with dust and wear.
Colored Grout
The use of colored grouts, colors other than gray and white, present design opportunities and challenges. I personally stay away from colored grouts as I believe they can look dated and as they wear and the color can change.
In the past colored grouts where very sensitive to the amount of water used in their preparation, so the end results could be unpredictable. This color shifting issue has largely been solved, but extra care should still be taken when mixing colored grout.
Boiling it all down
My personal views on grout are very traditional and perhaps too practical. I boil them down to four thoughts:
1. I’m not a fan of colored grouts.
2. White grout stains and turns light gray.
3. Black grout gets dusty and turns dark gray.
4. Do I want high or low contrast.
So, in my opinion, pick any gray grout you want, light gray, medium gray, or dark gray.