is it new? or already lived in?

Silver, gray, get a grout chart and try to match it.

Match the color to your Carrara marble; it really depends on the current lot.

Impossible to tell on a monitor. You get the gout samples and see what works with your tile best when it matches one of the darker colors in the marble IMO.

There's dozens of brands of grout, with different types undr their umbrella. And tons of "gray" grout colors. That translates to thousands of choices. A blurry bad picture is not going to track anything down. I could tell you that it looks like Mapei Cobblestone, but that might be too light in person, with a different batch of natural stone. Or it could be too dark for a lighter batch of stone. If Cobblestone works, there's still the question of which of Mapei's grout choices would work for that install.

Your tile pro or designer should be leading this conversation, with the material in hand.