Thank you all! Because we want a curbless shower, I have decided against a pre-cast shower pan. I considered trying to do something custom (corian maybe) for the shower area, but in addition to expense, that left the bathroom floor and the need for that transition from one material to the other to be waterproof as well. Which led me back to tile. And if I'm having tile for the bathroom floor, I've got the cleaning issue anyway and I might as well go with just one material to get the continuous look and save some money. I'd be willing to re-consider though if someone has information about making a pre-cast shower pan for a curbless shower work well with a non-tile bathroom floor. I'm definitely planning on somewhat dark grout. I'm figuring a medium grey tile and matching grout are going to hide dirt the best. But I also want it to be as easy to get really clean as possible when I do clean. Thanks for the information about ceramic tile working well for tight grout lines!

I was thinking slate, though it's a bit more blue than slate typically is, and I thought that slate was a sub-par choice for a shower?

Welsh slate (the dark blue stuff) or some pretty "light travertine look" tiles. It is WAY too hard to offer colour choices for flooring when everything is 'unseen'. Technically you can have ANY colour you want. The question becomes "what colour do you want"? And then "How dark do you want the colour"? Notice that both questions require a personal perspective....yours not ours.

Tilebar carries something similar. I used it on a clients bathroom. they're 24x24 and come in quite a few colors https://tilebar.com/rizo-tortora-24x24-terrazzo-look-porcelain-tile.html