Determine the location for the tile. Evaluate the color scheme that is present in this location and keep the tile style and color that you have chosen in mind. With all these things in mind, you need to decide if you want the color of the grout to blend in or stand out as an accent color. Virtually any color of grout is available by mixing white grout with color dyes. The shades depend only on the amount of dye added.
Decide if you want to stay with more traditional colors such as browns, black, white or gray. If this is to be the choice, then you must decide on the shade. One consideration that should be determined is what color will help hide the dirt and grime buildup that will occur.
Mix enough grout for the whole job and a little extra. When you are creating your own shade of grout to match or accent your color scheme, you can't count on being able to mix the exact shade of color if you must make more grout to finish the job. In this case, it is better to waste a bit on unused grout than to end up with two tones of grout in your project.