The primary reason for grouting tiled surfaces is to prevent water from penetrating behind the tile and weakening the adhesive or the backer and the structure beneath the backer. In mosaic artwork, the grout also has a visual function, and that is to contrast (not match) the tile colors. If the grout color does not sufficiently contrast the tile colors, than all the tiles blend together visually, and much of the “mosaic effect” is lost.
Grout Color Should Contrast Not Match
There are some novices who doubt my advice about contrasting grout color and even try to match their grout color to the tile colors. These are the people who later email me in a complete panic. They usually use the words “completely ruined” to describe what grouting did to their once beautiful mosaic, and from the pictures they send, I’m inclined to agree with them. (Note that these mosaics can be saved, but it requires either scraping the grout out with a grout removal tool or painting the grout with acrylic paint or some other ad hoc solution.)
A Medium Gray Grout
Since experience has shown time and again that the best grout color is one that contrasts tile color, the question becomes which grout color best contrasts ALL the different colors used in the mosaic. For MOST combinations of tile colors, the best contrast is usually provided by a medium to dark gray, with darker being the better guess if in doubt. Always keep in mind that the color of the grout will be significantly lighter when fully cured compared to how it looks when wet.
A Notable Exception: Lighter Blues
There are a few notable exceptions to the rule of gray grout being best. The most obvious exception is when you are using gray tile (duh), but the one that usually catches people by surprise is when tiles of lighter blue colors are used. Unfortunately, these are just the shades of blue that are popular for water and sky elements, so this is a significant exception. In this situation, a warm light brown or sand colored grout might be a good choice for contrasting the blue tile, but what if there are light brown tile used elsewhere in the mosaic? Is there a good standby color of grout for this situation? The answer is no, but there is a quick solution.
Go Look At Grout Colors With Your Tile
Building material stores such as Home Depot and Lowes usually carry about 30 or more colors of grout, and they have color swatches on the shelves and/or packaging so that you can pick out grout similar to how you pick out paint, only with much more limited options. The trick or tip is to not to try to do this from memory without the benefit of having your tile with you. Take one or two tile of each color used in the mosaic with you to the store and hold them up against the color swatches. I have even gone into the store with small mosaics, just as I have taken in parts of plumbing I was trying to match or replace. Don’t be self conscious about it. The people who work there are accustomed to seeing professionals at work, and you will be quite unobtrusive compared to the building contractors dealing with emergencies. At least you won’t be covered in dirt and holding a toilet seat or something like that.
Some “Advanced” Tips
From the many emails and pictures I have received in the past 12+ years, I can state with some confidence that novices tend to regret choosing grout colors as an attempt to add another color to the mosaic. Matching grout color to tile color tends to be even more disastrous.
If you already have your figures rendered in tile using a relatively small grout gap, and you like how those figures look, then your main objective while grouting should be to not mess up the visual art that was already working, especially if you are a novice at mosaic.
Of course, even a novice can take a few of each color tile and create an abstract experiment on a scrap piece of plywood and try a novel grout color on it.
The monochromatic nature of medium gray grout makes it contrast colors intrinsically, in the same way that back and white contrast colors intrinsically. All three are balanced in hue. The keep-it-simple and less-is-more principles really come into play when you decide to second guess some shade of medium to dark gray when grouting figurative mosaic artwork.
On the other hand, there are all those earth tones to play with…
Just remember to experiment on a piece of scrap before trying it out on a mosaic where 90% of the work was spent cutting and mounting the tile.