How to decorate apartment living room with black leather couch?

I would pick a new and "deep" color and get several accent pieces in that one color to place around the room. For example, deep red throw pillows, maybe a throw blanket, small decorative pieces to place on tables, all predominately red (or whichever rich color you choose). The reason I suggest a rich color is because if you get anything pale , it will simply fade into the rest of the neutrals (and you have A LOT of neutrals). It's the little things that pull a room together. If you cannot paint the walls, consider either buying or having a friend make a piece or two of artwork that go with your accent color.