Shower floor tiling

https://www.bhg.com/home-improvement/tile/installation-how-to/tile-grout/

Depends on how much you want the individual stones to stand out. Black grout will give a modern look but stones will not stand out as much. A gray grout will allow the stones to be noticed but will still be subtle. White will really individualize each stone. Personally I would use a medium to dark gray or a black grout.

Grey or black would look good. Have you considered that the pebbles might be uncomfortable to stand on? Happy decorating!

grey all the way

Will this be hard to keep clean?

I would use gray. Since it's darker it won't show wear, tear, staining, etc over time.

I think light gray grout would look pretty!

Personally, my choice would be gray.

I would go with dark grey or black

I'd use a very dark grey or black grout, be sure to use a few coats of a good sealer on the grout after it dries.

The gray would make the stones stand out and should compliment whatever color scheme you have in there.