We just did this in our bathrooms (still works in progress -- the grout just went in last week). In the master bath I used a darker brown grout in the accent mosaic tiles at the top of the shower and used a light beige grout for the rest of the bathroom (travertine tiles). Downstairs (not totally grouted yet) I again used the light beige grout for the field tile (again travertine) and will be using a bright white grout in the crystal marble mosaic accent strip. The master bath (which is all grouted) looks fantastic and I am so happy that I went with two colors of grout. I am sure that the downstairs will look just as good when the white grout goes in.

I'm thinking of doing the same thing. I saw some bisazza mosaic blends at Home Depot (yep!) that looked fantastic with carrara (the mix was white/grey with a few iridescent tiles included) and it wasn't too pricey ($7.92/sq ft). the blend was "roma plus" - I couldn't find that blend on the bisazza website, but HD lady said many mfgs. have special products just for HD. I was also thinking of doing something similar - using the bisazzas on one accent wall, and running a strip of bisazzas around the rest of the room. I'd pay special attention to the undertone of the subways and mosaics & make sure they're ok with the carrara. whites can be really tricky. when next to stark white, the bisazzas had a grey-ish cast, which looked great with carrara and the almost black vanity. don't know what brand of subway tile you're looking at, but i believe the various dal whites have slightly different tones. also, the glass mosaics tend to be thinner than regular tiles, so be sure to take that into consideration during the install. I've seen the mosaic bands set between pencil liners, and that looked good.

I'm also doing a similar color scheme in my daughter's bathroom. For accent, I am using a gorgeous Calacatta marble mosaic only in the niches (shower shampoo niches, and one recessed into the wall next to the vanity.) It's the same tile I used for the niches in my master bathroom, which has all white glass on the walls. Since it's on a vertical surface (just the back walls of the niches) cleaning isn't so much a concern. We've had it in the master bathroom for about 9 months and no problems at all. We are using marble pieces for the shelves, so the horizontal places where soap/shampoo might build up are easy to clean. You could also just use the subway tiles for the perimeters. I think this is a nice way to keep the wall tile simpler but also add a bit of interest and bling without getting too busy. Here's the niche by my master bath soaking tub, during the renovation:

I second Emmie's suggestion of Susan Jablon Mosaics. I bought 1x3 glass subways from her and I LOVE them. They were also so much more reasonable than any tile I could find locally. My tile setters were really impressed with them. I thought I'd have to just use glass accents, but at her price, I decided that I could do the whole backsplash in glass. I'll try to post a picture if my camera ever surfaces from all of our still packed boxes.