I just bought a brown sofa. It is the color of chocolate and beige, but mostly dark brown. I thought of red curtains but I don't know if that would look good. I also have a beige carpet and chair, so I need some colors other than neutral or brown.
been sitting on this for a few hours and you have some good answers here but will add mine, sorry if i repeat any. here goes - a bright brick red was the first that popped into my head and that was before i read all your question. light red. pink. dark or light b lue. ochre (orangey). light or dark orange. bright or apple green. lime. cream/beige/white bright or pale (yellowy). all shades of purple/mauve/lilac. all with or without brown/beige/cream stripes. again you could go with a floral pattern but i personally think solid or striped curtain would be best. in fact any colour you can think (except black) would be suitable. a lot will depend on the other furnishing in the room and whether you want to make it darker or lighter. try going to a number of shops and just browsing around and you may find it will hit you in the eye. good luck and have fun, i did thanks.
Burgundy is a nice color with browns If you want it dark otherwise go with light colors like white and yellows.
Brown goes with everything. It's kind of a neutral color. We had a brown sofa and carpet and chairs, and I used eggshell curtains, but I changed the valences to different colors. I also used a lot of colorful throw pillows in the room. Candles add color too. You can really use just about anything. Sometimes a plaid that picks up the brown is good. I just like white or off white curtains at my windows, for uniformity outside. Have fun decorating.
Your okorder / Good luck and have fun! Also get a few printed throw pillows with the curtain color in them and a throw for over the back of the sofa to tie everything together.