This is a partial view of one of my first mosaic projects - a wall panel that was about a foot wide and five feet high. The client wanted me to incorporate pieces from a box of costume jewelry and odds and ends that she had collected over the years. It was an intriguing project, because it evolved in a very organic and unplanned way. That process often works best for me, because when the finished forms are spontaneous, they have a freer look and feeling than when I plan something out in detail.
When I started out, I did not intend to grout, because of all the unusual shapes and forms that would make grouting very difficult. The piece includes sea glass, marbles, costume jewelry, and special pieces such as the glass bird in the lower left, and the glass leaf in the upper right, as well as tile and mirror in various shapes and configurations. The face is built up with mortar so it has some dimension. But when I was finished, I realized that the piece lacked cohesion, so I ended up "grouting" with paint - in this case a golden yellow color; I used a thin brush and - over many hours! - painted between each piece. (Since then, when I'm not sure at the beginning of the project whether I will grout or not, I paint the backing board first!)
Grout can make or break a mosaic work; and the color can either enhance or fight with the finished work. Usually a medium gray is a good choice, because it fades to the background. I use black if I want the mosaic to really stand out, and sometimes other colors if I want the grout to contribute to the final effect. Grout can be mixed with pigments to make any color you want. I recently learned of a good tip for deciding which grout color to use; it came from a fellow member of SAMA, the Society of American Mosaic Artists. Many of the books say to do test pieces, but if that's too much time and trouble (and it usually is for me, being someone who likes to work quickly), you can simply sprinkle a little powdered grout of different colors over your finished piece, and you will immediately see which color is going to work best.