There was much more fabric in the cotton curtains than I had expected, but it wasn't right for the couch: the cool blue went all right with the rug, but against the wall color it made the couch look like a jewel sitting in a shallow mud puddle. Also, the fabric is far too delicate for what our couch goes through.
Second try: I thought about what we have, and decided to try an old blue, gray, and off-white quilt. This is a commercially-made quilt with some hand topstitching that we have had for years and years. This quilt worked better than expected on the couch; the blue and the off-white go with the rug colors, while the shadows that the quilted texture creates in the off-white fabric come close to the muddy deep beige of the walls. I feel like I am starting to get somewhere with this room.