According to Constellations of Words,
The word congruent comes from the Indo-European root *ghreu– ‘To rub, grind’. Derivatives: grit, groats, grout, gruel, grueling, great, groat, gruesome, chroma (color), chromatic, chromato-, chrome, –chrome, chromium, chromo-, chromosome, gravel, congruent, congruence. Pokorny 2. ghreu– 460. Watkins
It is only fitting that grout is relate to “grueling” in origin, since there are moments where grouting appears to be a crazy idea. After making my chocolate truffle mixture, and in my nitrile gloves, I scoop up a handful of grout and begin to cover my mosaics. Some mosaicists use a grout float or other tool for distributing the grout among the interstices, but with the variety of textures and heights of the tesserae in my work, I find applying it by hand to be most effective. As I massage the grout into the gaps, all color is obliterated in a muddy coating. This is a leap of faith. Covering all the carefully glued pieces with a type of cement is counterintuitive.
Fortunately, I’ve grouted enough pieces to be somewhat adapted to the moment of transformation! But even now, there is a realization of the point of no return. Grout can bring a mosaic together, like its cousin “congruent,” or it can be divisive. A dark grout unites the dark elements and breaks apart the light ones, and a light grout does the opposite. This mirror has a medium tone that is fairly uniform across reds and oranges, so the brown grout is like a setting for jewel tones, the black velvet under the sparkly ring. Grout is about relationships between colors, and can change the perception of the tiles, drawing them forward, pushing them back, intensifying or diluting the color.