Definitely not brown. I chose white from your vote, but, truthfully, I would never put in a white couch, at least in my household. Yours may be different. What color gray on the floors? Is it a light gray? Then maybe a charcoal gray sofa. Is it a dark gray, then maybe a light gray sofa. I have light gray walls with one dark turquoise accent wall. Since my couch sits against the turquoise accent wall, I have a light gray couch. My floors though are oak colored, but, there is an area rug in front of the couch, so, it really wouldnt matter much if they were a gray color.

Your palette sounds beautiful thus far!

Depending on the specific shades of each of the colors in your space

flooring, white would be a safer color but brown will add a richness and warmth to your space. If your flooring has some warmer tones in it, a brown sofa will likely compliment your flooring. If your flooring is dominantly cool tones, brown may clash. Another color to consider may be something in a smoke or mushroom tone:

It'd be a great mid-tone neutral which would work well with both warm and cool finishes.

Best of luck with your project!

White would be nice I love my white sofas in leather they have stood up to hundreds of parties big dogs and kids for 11 yrs and still look like new. They will allow you to change wall color when ever you want and accessories. BTW I dislike brown sofas and it seems like it is always the choice in the US there are literally hundreds of colors of sofas and brown would be the last choice for me or my clients. As for drapes choose a rug first then a color from the rug or just keep it simple and do a nice heavy white drape or if privacy is not a big issue just really full sheers. I would be careful to not add much more teal in the space but find another color for accents.