Usually you want the grout to disappear, so you choose a color that's as close to the tile color as you can. Lately, I've seen a lot of people using darker grey grout on white subway tile. I can't stand the look, personally. To me it just looks like your grout came already mildewed and dirty. Remember in the late 80s, early 90s, when all the houses had white tile countertops and they used the dark brown grout? I imagine it will become as ugly and outdated as that fad did.

Go to home depot, Lowes or where ever you bought your tile and look at a grout chart. Pick out a color as close to the tile as possible so you can have a "seamless" look. Are you installing this yourself? If not, you need to tell the installer what you have chosen. Sometimes they have a certain brand of grout they like to use and you may not like it. You pick it out---its easy!!

Presidio...I'm with you on that. The floor tiles in both my bathrooms are 12"x12" white tile with dark, dark, dark grey 1/4" grout joints. It doesn't look dirty, but it shows every bit of minor misalignment of the tiles (not that anyone but me notices).

Fortunately the wall tiles are white with white grout. But at night, with incandescent light, you can see that they used 2 different brands of white tile around the whirlpool tub; the colors are the same during the day, but different at night.

As previous person said ,go to home depot or lowes if your doing it yourself.

And pick up a color chart.

If your working on some thing that already has grout as in your case,you will need to scrape it back,to allow room for new grout on top.

seal grout a few days later.

Ed

thanks everyone for your advice! We are ordering the tile from walker zanger and its kind of a mish mash of different colors, a light green and medium green to light white, its the pasha green river onyx mosaic - should i choose the lightest color? i guess i should choose something with a green tone to it or is that too much matching? the catalog says there is wide variability (i still have not received all the tiles, just the sample) and also says this "White thin-set or mortar is required.
This is the case for all limestones,
tumbled stones, “antiqued stones”
and light colored marbles." We are having our contractor install this but not sure if he has worked with this type of stone, is this something that we need to have a tile specialist install? thanks again