My battle against beige

I'm super excited to have my own office/gym/be by myself room at the new house.  It has a floor to ceiling bookshelf for my books and picture frames and plenty of room for my desk and a workout area.  However, it's a whole lot of beige walls with tan carpet that we won't be replacing for another year or so.  That means I need to find the right rug to jazz it up and keep it from becoming depressing.

My budget is low, and I want something that pops but not too crazy that I can't use it somewhere else one day.  I've been looking at some of the ones below and will hopefully be able to order something this fall once we're married and settled and I can devote more time to the house.

So far, the large rug on the bottom right ($139 at Target!) and the red zig zag are in the lead, but I change my mind a lot. 

Which is your favorite?  Or can you help me find a modern green rug that doesn't look like it belongs in an old lady's house?