I'd simply match the white counter with grout. Or match the tile itself. Do a sample board first, let it all dry and make sure, before you grout the entire splash. You can't " warm" it with grout that highly contrasts, especially if dark. It will just end up busy busy busy. The fact is, the shelf is putting the tile in shadow, as happens with upper cabinets.

What also MAY be making it appear gray is your window trim, which seems a bit yellow by comparison.

I think it looks fine. You aren't done yet ..........so I will hope the light over sink is being replaced? Doesn't seem to suit the rest....: )

If you go light or white do not pick epoxy. It will yellow. A urethane grout like this Bostik Dimension Urethane Based Grout (9lbs, 691 Polar White) - - Amazon.com is an option you may have to recommend to your tiler. It needs to be wiped sooner than traditional grout. Tell him that.

If you use a grout that contrasts with the tile color, it will accentuate the non-perfect nature of the tile & setting. Tile especially over the range has varying grout lines which will stand out with contrasting grout. Depends on how you want it to look. I'd go with matching grout in this case.