That tile adds dimension to vary the color reflection affect. I wouldn't choose charcoal. It'll damp down the affect. . . .You could get a sample card to view the options in your home and lighting. I'd try urethane grouts like these from Bostic. https://www.bostik.com/us/en_US/markets-applications/building-construction/wall-floor-installation-systems/tile-grout/pre-mixed-urethane-tile-grout/ 

Easier than epoxy grout and also waterproof and stain resistant.

https://www.bostik.com/files/live/sites/shared_bostik/files/Documents%26Brochures%20(new)/United%20States/Brochures/C%26C/bostik-grout-sealant-colorguide.pdf

I'd check out one of the Dimension colors. They could add to the tile affect.

Let your installer determine the type of grout to use based on your material. Now, as for the question you asked, I'd stick to the closest match to the tile. Have the installer do a mock up of a few colors if needed so you are comfortable with the choice.

why gray? if you want a punctuation type effect , pick black and place a black glass art piece on the top.

Thanks all, I contacted the tile store and they recommend a light grey (I’ve attached their mock-up). I don’t think I like it very much so I’ll keep looking

Your tile has a shimmer to it why not use a grout the same color with shimmer as well

The Bostic Dimension grout ncludes material to add shimmer.

I don't have a grout recommendation but wanted to share that I love that tile!

seems like the gray they show is to make the tile look like small stone pieces. Cuts and angles like naturally occurring pebbles....? so maybe do that. It wasn't clear until you showed it close up. Its not like flat tile.

They do make gold grout: https://tiledoctor.com/gold-grout/

This looks like Porcelanosa mosaic tile. I have it installed in the platinum color and we used their clear grout https://www.porcelanosa-usa.com/products/building-materials/grout-caulking/epotech-crystal

You can also look into the grout recommended on their site https://www.porcelanosa-usa.com/products/catalog/product/view/id/12739/s/gravity-aluminium-cubic-gold/category/16/

I love this tile! Can you share after pics? And I do like the idea of the black grout.

I would use it, with the black grout, but only as a backsplash. Less is better for this install.

Over top of cabinets I would paint the wall in the niche something like this and add art. You have two lights over head that would make this work.