Selecting a grout color can often be the scariest part of any mosaic project. It can make or break your final result. Like framing a picture, your choice of color can enhance or compete with the finished piece and can emphasize or de-emphasize the design. It can also "show off" your mistakes in laying the tesserae and hide things you want to enhance. Taking a little extra time and effort will pay off in the end.
When in doubt do a test board. Glue up a sample board and grout it with a few different choices. Usually, this will make the choice clear. Another quick test, most grouts “out of the box” will cure to the color in the box (pre-water addition). Try sprinkling different areas of the mosaic with a little dry grout in the colors under consideration. This gives you a good sense of what the final grout color will look like. Once you find the right one simply brush it out.
White or light-colored grout can be very intense and it is rarely a good choice unless your piece is white or very light in color as it tends to fracture the mosaic. It does work well with broken China (Picassiette) mosaics when the china is light to white in color.
Black or charcoal will intensify the color of your mosaic. It often works well with brightly colored tesserae.
Medium gray is often what I call the "when in doubt choice". It tends to recede and blend the colors together. Gray will work well with almost any colors of tesserae. Sandstone colors also make a good neutral choice when you are looking for a warmer tone.
There are many other premixed colors are available and make great choices don't be afraid to give them a try.
If you want a special color that is not commercially available you can use powdered or liquid universal pigments. They can give good strong grout colors. Acrylic paint is not recommended as a colorant. It takes a lot to effectively change the color and can cause cracking and a strange plastic-looking surface to the grout.
When using liquid or dry pigments in almost all cases no more than 5% pigment should be added by volume. Using more can affect the stability and strength of the grout. If you use liquid colorants remember that liquid replaces some of the liquid needed otherwise, adjust accordingly when mixing.
It is also important when custom mixing colors to make enough to finish your entire area. Unless you have done some really precise measuring and logging of those measurements you will never be able to create an exact match twice.
This YouTube video demonstrates in detail how to custom color grouts and thinset.
Coloring Thinset mortar and Grout Video
https://youtu.be/KB9sGBZyA7s
