This isn't something you would want to model in Revit for a documentation purpose, its overkill, takes too much time, and has a potential to bite you in the butt if the product, size, layout, etc. change in the future with VE, Design Changes, or ownership direction.
"Proper" (or at least industry standard) way to handle this would be a model pattern as @Anonymous said, then covering you material choice with specifications or materials appendix, schedule, etc.
Looks like you are a student based on your past posts so I assume you are doing this for an assignment and maybe a 3D visualization or rendering...?
3D hidden line camera view: You could include the grout lines in the model pattern. This is normally overkill for normal 'documentation' but is sometimes used. This would show up the grout in a 3D view camera though if you wanted.
Rendering/Visualization: You would use the material Appearance to control this using the bump and other channels to achieve the 'grout' look with an image representing your coloring and such. However, revit is a terrible rendering program, so keep that in mind.
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