Do you have a rug? Do you want to stay in the black/gray/white scheme or are you open to color? Also that is more loveseat than couch and because of the small size I think I would skip pillows and use a throw for color. The throw won't have to be moved every time someone wants to sit down

I like jck910's idea of a throw but would still add pillows, but lumbar pillows, to add some color. U living room is gray too, I took an accent color from a picture and added a gold floor lamp

That sofa should have no pillows where can you even sit there a throw for color and texture is the right choice.