This e-mail just arrived:
For starters, what color grout to use on your back splash is not a huge dilemma. Deciding to take a loved one off of life support is. Let's try to work on getting some perspective.
For seconders, your dilemma would be solved best by the designer you're working with or the sales person you're working with where you bought that tile.Posting photos on Houzz and asking me for advice on grout colors I can't see is how you end up in real trouble.
If you're working with a designer or a reputable salesperson, he or she will ask your installer to do two mock ups. Each will use your back splash tile. One will have your tile with Pewter Waterfall gout and the other will have Silver grout. Once you see how those two different grout colors affect the color of your tile in your own home your decision will make itself. Do not buy tile from someone who won't do a mock up for you.