I have a white subway tile backsplash with white grout and my cabinets are white. I did have one aqua straight line of tile about 2" wide running through the middle of the tile, but it still looked kind of plain. I found this item on Home Depot website, they didn't stock it here. I ordered it online and picked it up at the store about 3 days later.
It took about 2 hours to do my backsplash. Just try and get it on the grout and have a wet sponge handy and wipe it off the tile rightaway. It came out really nice, I think it made the tile really pop. Don't worry about it getting on the tile unless it's something porous, subway tile as you know is very slick and smooth so the dye didn't stick even if I wiped it off 5 minutes later. I used a dark gray, I thought a color or black might be too drastic a change.
Since you're dealing with new construction, why can't the builder just use colored grout to begin with. That would be so much easier and if they don't want to hassle with it, tell them you'll provide the grout. Then go and buy some. Coloring is an OK method once the tile is set, but I don't understand why it can't be done initially.
Home Depot Tile Dye