Zak: A more masculine color scheme could be all your bedroom needs. Darker colors are generally more masculine, particularly olive green, navy blue, dark brown, and other earth tones. Black and silver are also good colors for masculine decor. You don’t need to paint in order to transform the color scheme of your room. Instead, you could buy some new sheets and blankets (a good investment anyway), a lamp, and maybe some storage cubes. You don’t need to spend a lot of money, just get a few things that establish a theme in your room.
I don’t know what the average 20-something guy’s room looks like, but in order to make your room look like that of a fairly fashionable 20-something guy it would probably be advisable to avoid hanging posters on your wall unless they are framed as well as avoiding posters of scantily clad women (or men), cars, or anything that looks like it belongs in a beer commercial (you’re in your 20s, not your teens). Probably the most important thing, though, is to make sure your room expresses who you are. Display pictures of places you’ve been or any neat photography you have, art you like, or mementos you think would look cool.
Most articles I’ve been able to find that relate to this don’t seem to be for the ordinary or average 20-something, but you might be able to get some inspiration from SPACES: Masculine Design Inspiration or the masculine bedrooms tag on Pintrest.