





I have people that do a piece where the focal point is all in blue and the back ground is all in brown. Since blue is a cool color they think gray but imagine gray grout with brown tiles...Yuck! You want to choose a color that will make the entire work flow. Brown would be best in this case. What if your focal is all blue and the background is all orange. In this case you will want to use gray to keep your focal from being fractured. Gray would look fine with the orange tiles too.
Don't forget the correct intensity. If your piece is mostly dark then use a darker tone. If it is mostly light use a lighter.
So the rule of thumb is just a guideline. Always do the grout test to make sure of what color to use. Simply don a mask and sprinkle a little of each grout possibility into the grout joints of your piece. See what looks best then vacuum the dry grout out before mixing the real batch.
Although you can lay tiles on your edge, I never recommend it. If your piece gets bumped or falls those edge pieces have a good chance of falling off.