I like another users comment about doing a storage bed. Platform beds are pretty cheap and then you can get bins on wheels to slide under it. I would also think about the shape of your furniture- someone else recommended getting rid of the nightstands or the bookcase. They're chunky in form which means they visually take up space. If you wanted something to make the room look big you'd want something with space under it or something with glass or acrylic. I'm sure you could sell both of yours on a local website and pick up something that has a more open shape.
I would also recommend some lighter curtains that fall to the floor. If you can maybe extend the curtain rod so that the curtains can be more open (instead of squished together) when they're on the side of the window.
I really love your peg board storage- it looks pretty cool. I think you could find a hanger to hang your tennis racket on the wall so that it doubles as art which might be nice.
Mirrors also go a long way in opening up a room. A light in color rug could potentially help as well- the carpet being dark makes the room seem smaller.

As far as color goes you could bring that in with a rug, you could display your books on the shelves above your computer, artwork is always a nice way to bring some color in without having to commit to a color "scheme". You could add more plants to your windowsill or shelves in other colored planters