NT is the queen of multiple grout colors on one piece, so no doubt she'll pop in here. I've not done it, but in reading of others' techniques, taping off the areas seems to be the way they do it. Re color - I switched to charcoal for everything more than a year ago, and have found my favorite and w/stick w/it. Occasionall, though, I use black. I like the contrast w/black, but like the charcoal that shows up most everything well.

You can also do a paint wash over the grout after you grout also, use slightly watered down acrylic paint, and it will wipe off your tesserae. its much easier to darken grout than to lighten it. So if its a small area you are worried about painting might be easier, but aside from that taping it off.

like everyone said, use painters tape. My advice from past experiences - do your darker colors first, go through the complete cleanup and then tape it off to do the light colors. The reason is the dust from one color will stain another, would be a serious issue with white. Doing graduated colors is not that big a concern since some blend is okay.

and yes a paint wash can save your butt sometimes, never rule it out

Thanks so much for your advice, I'm going to think about it for a few days and then jump in. My initial thought is to go with white and then paint, seems so much easier! I'll post a pic when I'm done.
Mattie