A powder-room floor isn't very likely to be wet (well, one hopes not anyway!) so slipping shouldn't be much of a worry. If you're concerned, you can choose tiles that have a higher "coefficient of friction." Coefficient of Friction is a measure of slipperyness and ranges (in theory) from 0 to 1. Zero would mean no friction at all while 1 would be mean you couldn't slide against the surface at all no matter how hard you tried. For comparison, rubber tires on dry road surfaces experience a COF of about 0.7 to 0.8 but when the roads are wet the COF drops to around 0.4. Wet ice can have a COF of 0.1 or even lower. For a shower floor, I would look for a tile with a wet COF of at least 0.6 or else use 1x1 or 2x2 mosaic tiles so that the COF of grout is more important than the COF of the tiles. Even with 1x1 mosaics though, if the tile is very slippery, you can wind up with a very slippery shower floor.

I don't have a picture (I can grab get one if you like), but I did the same thing in my current project - new wood floor in the kitchen going right into existing in the dining room. The floor installer feathered some of the new boards into the dining room floor so it looked continuous, then I had all the existing floors sanded and refinished and stained the old+new 1 color. The result is a transition that looks completely continuous and uniform. Of course I had to refinish all my existing floors to do it. If you don't want to do that the only thing I can imagine working is a perpendicular strip to edge the new and old. It'd be impossible to match the old stain with new perfectly so you'd need something to separate the two. I have a spot in my house where the boards are mismatched so whomever installed it back in the day put a strip. I attached a picture of that.

deleted because duplicate This post was edited by elphaba on Wed, Aug 7, 13 at 0:08

Is it a flaw or a feature? Make lemonade from the lemon... If you were able to put an attractive decorative boundary on the blue rectangle in a darker wood, the wood inside wouldn't abut anything else and could be CLOSE or CONTRASTING in look to the nearby wood. If you ran TWO contrasting colors outside the rectangle, that would be swell. And if you had some kind of fab inlay, you'd be thrilled every time you walk through! Your space is small enough that you might contact a vendor about remainder materials. Never know what is out there. Here's a specialty company whose stuff is in a local showroom by me. I have really liked their restraint and good design on many items. [Remember to wipe up after you drool on the wood floor.] Oshkosh Designs of Wisconsin. Click on the categories at left for more choices. Can't afford inlay? Another option is to stencil the floor with paint. You can duplicate many inlays this way, an old historic option for those of the middling classes. Here is a link that might be useful: Czar Floors, if you want to go all out...