Depending how vivid your scarlet is: You can go with white or cream so it really is the accent wall and nothing is shouting for equal attention You can also go for a muted grey or beige

For start I would get rid of those blinds.it would open the space up.I would get those linen drapes in cream color and put them next to each window.I would also put a couch looking toward the windows and in right corner i would try and get a media table and put tv on it.maybe get a nice carper (jute is nice with dark floors)and I would get some nice chandelier maybe industril or modern looking and some small table to go infront of the sofa.this room has a lots of potnetial.do not give up :)

I'd suggest dark red curtains if you need curtains at all -- the windows are beautiful as they are, and I'm not sure whether some kind of blind might not give you the ability to darken the room when you want to. On the wall, if you are real fans of any movies or TV shows, three big posters would be fun. Otherwise, I would still suggest using three pieces of art there of that size, all equal in size and somehow relating to each other. Three floral works, or three works in the same style, perhaps. I think a large plain textured rug in a light neutral from one of the cushion fabrics would tie the space together well. You might also consider a lovely low coffee table on the rug to lay out your snacks and drinks for serious media nights.

Consider Grey or white. U could also use geometric or any pattern in mint green