I love these pillows! What a great color and pattern! With this kind of palette you have many different options. If you are a colorful kind of person you can add a touch of yellow in a solid or maybe even a stripe for your draperies. If you are not so much into adding more color then gray can always be an option. I would go with a light gray since your couch is so dark.

If you are looking to add one more color to the room, consider brown, yellow, or gray.

The picture of the green drapes posted by Maureen really make the room. The white and blue are more subtle but I like them too.

Green is considered a neutral color as it is the color of leaves, the background for flowers so you have some options on color. Because the sofa is so dark, a lighter color would make a good contrast. A light blue or light green or yellow would look nice. Try a lot of colors even pinks. Good luck.

I like the way the green coordinates in the room.

The pillows on the sofa unit are really nice and they can go with other patterns nicely, If you want to go with a solid green color or blue that would work as long as it is lighter than your sofa. Depending upon the look of your whole room, you could go with an off white or beige with a dark blue or green pattern similar to the pillows.

Be careful with the greens though. There are many shades and you have warm tones and cool tones, so look at them together .
Here are a few examples below.

I would like a white linen drape with a band on each side of the color of your sofa.

You could pick up the green in an area rug.