We have a marble mosaic backsplash we we picked a grout color that blended in well with the backsplash so as to not detract from it.

I have two colors in my backsplash tile and I chose the grout color for the lighter and more subtle color and have been happy with the choice. You should be able to get a sample of the grout in a piece form(hard) and look at it next to the tile in the room.

DH and I prefer grout to blend in, so chose grouts that matched our tile as much as possible.

We matched the grout color to the countertops to help the color pull into the backsplash. Lots of options and possibilities ...

We wanted a period look that would be low maintenance, so chose an appropriate grout color for that---it also happened to pull in the counter, cabinet, and floor colors, but that was just a happy side effect.

I took a sample of my countertop to the tile store and chose a grout color similar to one of the colors in the countertop.

I had a hard time picking a grout color so we did up a sample board. We tiled a few extra pieces onto some plywood and then experimented with grouts so we could be sure we liked the color we picked. It's not hard to do and it was very helpful. If you are wondering how the grout color will dry vs the samples in the store, I followed the advice that the color of the grout in powder form will be pretty close to it's color after it is applied.

Oh yeah, we did the sample board too. To avoid having to buy a whole bag of grout in a color that didn't work, we bought the color-matched caulk (cheaper since our grout color only came in a big bag) and used that on the sample board instead. Worked perfectly (and we got lucky and liked the first one we tested!) The caulk for ours was a pretty perfect color match.

We're going to be making our backsplash grout decision shortly. What kind of tile are you using? We have ivory subway tile and we may choose a slightly contrasting grout colour, possible gray, to emphasize, rather than minimize, the pattern a little. We like the way the tile looks ungrouted and don't want the shadow lines to disappear altogether. I'm going to post a new thread about our backsplash tile (which we're installing ourselves). Artemis, we'd like to hear more about your grout matched caulking. Where did you get it? What brand did you use? And anything else you could tell us would be great.