" I know I want a medium brown grout in the shower "

Why brown grout with white subway tile?

" or do y'all think the mosaic hex would look good with a medium brown grout? "

NO!

I would use a white grout or a very light gray all the way around, no brown (medium or any other).........

I like contrasting grout lines with subway tile but don't want to go as dark as black.

from CoPilot AI:

Q: "What's the best grout color for small, white mosaic shower floor tile?"

A: "A light‑to‑medium gray grout is the strongest overall choice for a small, white mosaic shower floor. It keeps the floor looking clean, minimizes visual clutter, and avoids the maintenance problems of pure white grout while preventing the heavy, busy look that dark grout creates.

"Best choice for your specific scenario: Light Gray

• Provides subtle definition so each mosaic piece reads cleanly without making the pattern loud.
• Hides the natural aging and mild discoloration that happens in wet areas far better than white.
• Avoids the “busy” effect that dark grout creates on mosaics, which can make a small shower feel tighter.
• Recommended by multiple tile‑installation guides as the “safe neutral” for white mosaics."

I'm confused as to why you want to use a medium brown grout on your shower walls . . .

I'd use Mapei Warm Gray for both the walls + the floor.

To answer your question about using two different color grouts for the shower, the floor tile and the wall tile will be grouted at different times (at least both of my showers that were renovated in the past few years) - so, I don't think it matters to the installer. You may have to pay for the additional bag of grout due to a second color.

I used different color grouts for the tub/shower surround walls and the bathroom floor in my upstairs hall bathroom. I also used two different color grouts in my primary bathroom (a different color for the tile used on the vanity wall vs. the bathroom floor/shower floor/shower walls). My installer didn't charge me anything extra in order to change colors.

Mapei Warm Gray works well with many different tiles.

This is the bathroom of a pro here on Houzz (Beth H.):

She used Mapei Flexcolor CQ grout in Warm Gray on the walls/inside niche + Mapei Charcoal on the marble hexagon shower floor tiles. She did very thin grout lines - the warm gray is a subtle color.

Here is Mapei Warm Gray with a similar color mosaic tile (the mosaic below is made out of marble):

Warm Gray was used to grout the back of the niche - a white grout was used for the subway tile on the walls.

Yeah, I'm going back to warm gray. I just have too much time to sit around and overthink. The work will start in about a week and a half so I need to just sit tight.

Thanks, all!