Grout color choice may seem like a part of your renovation that may not seem important, but it can really make a huge difference in the overall look of your design. Black grout can be really graphic and edgy way to punch up classic subway tiles. It is also the way to go if you don't want to find yourself scrubbing as often. Look at how the subway tile pops in the bathroom below and also balances well with the black cast iron tub and brass accents. Its contemporary rustic, which is a great way to work a black, white and brown color scheme.
On the other hand a light gray option can be a way to make your subway tile look seamless, blending into the canvas rather than popping out. See how this bathroom below with its light gray grout makes the tile float into the background. It keeps the look fresh and crisp, and the chrome/silver hardware can also keep the look classic. If you want to punch up the look without going the edgy black grout route, think about changing the hardware to brass and punch up the walls with some color, try Hermes orange with navy and white striped towels or bath mat.
I think it is a matter of looking at the overall design of your project. If you are working with a bathroom, what color are your fixtures and hardware? With a kitchen, think about the grout in relation to the cabinets, flooring and paint color. When in doubt you can always create a sample of tile with each grout color and live with it for awhile. I love that designer, Amy Meier, choose the black grout in this kitchen below because it works with the mix of those cobalt blue accents, dark countertops, and classic white cabinetry.
Think about the way you wear black and white in your wardrobe. When you make that choice to put either color on you know that either way you really can't go wrong!
Which do you prefer? Light or dark grout?