I am getting requests from people lately trying to match a previously installed natural stone flooring several years after installation. Let me tell you, that is no simple task. Stone is natural product. With that, the tones, shades and general patterns will change with the location within the quarry. The variation is part of what makes natural stones such as travertine, granite, marble and slate so great. It gives them character. The real problem comes in when you try and match it years down the line. The raw materials will almost certainly be from a different location within the quarry, if not an entirely different quarry.
With all of this in mind, the moral of the story is this. If you think you are going to need your job to grow into another room down the line, order enough for that area at the same time. Also, for repair purposes, I always recommend trying to keep a few extra tiles on hand. That way, should anything happen to your tile, you can replace it with something that should be the same. Trying to match after the fact can be an extremely frustrating endeavour.