While I don't love the charcoal grey grout on the white subway tile in my recent home purchase, I can't say that we've had any maintenance issues with it.

It won't show dirt and so is easy to maintain, but any time you have a high contrast difference between the tile and grout color, you want to be very, very certain that the tile is laid precisely. It shows every little flaw in a tile job.

Dark grout will also show soap scum and mineral deposits from the water, if your water isn't pretty soft. I don't know about using chemicals to clean it; it will still grow mold, but you won't be able to see it. If you use a cleaner with bleach to kill the mold, you might affect the color of the grout - I don't know that, but would be concerned. If you do go dark, maybe have them use epoxy grout. It can take any kind of cleaner and also does not harbor mold (in my experience).

Get an excellent tiler, especially if you use dark grout. White or light grout hides any small imperfections better. I’ve had both light and dark (dark gray, not black) and they clean the same. I have hard water. Best of luck!