Go gray it will look beautiful...it will make the subway tiles more prominent...what is the style of your bathroom?

Thank you. The style of the bathroom is modern. The layout of the tub/shower is like this photo (but tub surround will be the floor tile - the floor tile is similar to the one in the photo too)

Contemporary and Bright Twin Bathrooms · More Info

Contemporary and Bright Twin Bathrooms · More Info

I'd stay more toward the tone of the tiles you are using if you want a sleek more modern look.

Adding a darker gray does give a more Farmhouse style but if subway tiles are stacked instead of brick pattern they look much more modern And you can add gray grout without it looking to Country/Farmhouse.

https://happy-floors.com/products/apollo/

https://www.msistone.com/quartz-countertop/carrara-marmi-quartz/

There are grouts that have additives in them to help with your concerns. Ask which lines the installers use and pick their top version; they all have an epoxy version.