What color(s) to grout a mosaic is a common dilemma. And a personal decision each artist needs to make based on their vision for the finished piece. This post is just some of my thoughts and by no means comprehensive.
I have a general rule that often helps me make the choice:
blend the background, fracture the focal.
But, I break this rule regularly. Usually on realistic pieces when I feel that all the careful shading I have done on a focal will be lost if I choose a grout color that breaks it all apart.
That was the case with this little mosaic I did recently:
I even briefly considered skipping the grout on the kitty herself but rejected that since I could see the substrate through the spaces. Additionally, my glue was
weldbond which I don't feel would be lasting in Florida humidity if not protected by grout. As an aside- I am looking into
smalti as this material is not traditionally grouted.
So, for kitty herself I actually used three colors of grout. Tea green for the irises, black for the nose and right eye pupil, and a natural grey for the fur areas. The natural grey is in between in tone to the light and dark fur tones and provided subtle contrast so she is still clearly mosaic but the pieces blend.
For the background of kitty, first decision was glass color and flow. I chose the pink as a compliment to her eyes and to emphasis that this sweet kitty is a she. The squares are a contrast to the irregular pieces used in the fur. Because of the blending of kitty with her grout, I felt I needed a grout color in the background that did not completely blend but that provided a very
subtle contrast as well. Warm grey was my choice here.
To help with my decisions, there are a couple things I do.
First, I keep a box full of little cups and their wood stirrers that I have used previously to mix up grout. I label both the cups and sticks and use them to do a squint test.
Second, I will sometimes sprinkle a small amount of dry grout in an area to test the effect. The color of the dry powdered grout is about the same as cured grout in a finished project. You just have to remember to use safety precautions as you do not want to in-hail any of the grout powder!
There are a couple of online resources that can help to visualize different grout effects too:
Mosaic Mercantile,
http://www.mosaicmercantile.com , has the
grouterizer.
Choose the mix of color tiles similar to your mosaic, 16 choices, then choose from ten grout colors to see their effects.