I would use a clean large, white tile for the floor. with gray or black I would be afraid of making the room look dull and gloomy.

Would you consider black, dark grey or white penny rounds? I think white would look great but then you'd have to go w/ a dark grout probably, being that it's a bathroom floor + all.

What Alyssak says.

white penny rounds!!

I'm with Alyssak and Molly -- if you don't want to go all white, you could do black or gray and white (since you mentioned that you liked those colors) or even black/gray and white with accents of green similar to the existing green tiles in the shower in a border to break up the white -- could use gray grout to make upkeep easier, too.

I think a dark gray, slate colored tile would look good with the green. maybe paint the walls a brighter white and the bathroom won't be too dark. or white would work as well, but ditto the dark grout comment. or if you want to be more adventurous you could do a mosaic tile that has some similar green shades in it.

What medusa12120 said - though I'd look for a light grey or white marble with a little greeny tone in the marbling it to go with the existing tile.

Actual slate would probably look really good---you'd have some variation in the color from green to grey, with streaks, depending on the stone you choose.

As soon as I saw this tile, my mind went slate. When you couldn't decide between gray and black, my mind screamed slate. Many of the slate tones would compliment this tile very well, and would work great with just about anything else in the bathroom (would work with just about any vanity - wood, white, or black. I assumed the vanity wasn't that crazy, since it wasn't included in the question).

I doubt it would look dull or gloomy, and it would compliment the shower curtain and go with some of the natural vibe I'm getting from the picture. Go slate! :D

Slate.

PS - I'm not sure some of you folks read the question well. He is inquirying about the floor ONLY.

Oops -- "She" is inquirying . . .

I agree with some of the statements above, I think a slate would look great. However, since slate is a relatively soft stone and tends to split/flake, I'd go with a slate-look ceramic with a black, blue-grey and green movement in it.

I LOVE the look of the green tile!

I'd do either dark green slate, or a randomized combination matte and shiny green pennny tiles in the same shade or lighter shade.

Love that tilework!

I highly recommend Heath Ceramics out of Sausalito - I think that another larger-format rectangular tile, laid in a staggered pattern, but in an off-white would look perfect with what's existing.

http://www.heathceramics.com/go/heath/tile/

What other colours are there in your bathroom? How big is it?
Have you considered yellow tile or light green?

Large, white/off white matte tiles. Dark tile is difficult to keep clean in a bathroom.

Another vote for white/light grey! Although slate would be lovely as well, I love the clean look white gives.

I think this octagon dot design would look awesome and replicate your vintage feel:

http://www.daltileproducts.com/series.cfm?series=85#496

I think either white/black or white/grey would look awesome

I think you could go for a natural looking slate that has some browns and greens in them that will pick up some of the dark green from the existing tile.
At Lowe's:
http://tinyurl.com/3cb8fdx
http://tinyurl.com/3jd9oce

Matte black penny tiles

I think something creamy would look super nice!

I think a larger-format light gray tile would look nice, or a white...but I wouldn't go any darker than a light gray or you risk darkening the whole room.

Get some samples from your home improvement stores/stone yards and lay them down. I've gotten some free samples from slate yards. That way, you can mull over what you like vs what works, etc at different times of the day. Have fun!

We have a similar green tile in our 1955 house. We loved the retro feel of it, so we just changed all the fixtures and the floor. We used 18 inch pure white rectified tiles and it looks great. White tiles are definitely harder to maintain, but it was the best fit for our green tiles.

Light gray 6x18 matte porcelain with white grout to match the existing tile.

River rock

For my contrarian view of the day:

Cork flooring!

It was good for Fallingwater, it would be good here!

Although, a relevant question is -- what color are your fixtures (tub, sink, toilet)? I'm assuming white with the cork recommendation.

Check out House Beautiful's bathroom of the month in the current issue. (Their website is inexplicably BAD, so pick up an issue in a store and flip to the back.) It has green striped wall tile and round white floor tile, and I immediately thought of your post. I applaud your use of the existing tile!

Thanks all for the suggestions - we love the idea of slate especially. Will let you know how it goes! You guys are great as always.

I think white square tiles or large rectangular white squares. I think you need something light and larger to balance out the small dark rectangles and more dark tiles on the floor would make the room feel gloomy. I think dark bathrooms can work in some cases but in this case I think that shade of green with black or gray would be too much. I also think white would keep it more modern looking. That's my two cents but do what you'll love!