What color are the walls, floor, and trim?

It really depends on how you use your duvet. Is it a bedspread that goes over a top sheet and other colors? They it can be anything you want. Do you sleep under it without a top sheet? Then it needs to be machine washable and changed when you change your sheets.

It's difficult to see how large this pattern is, but if you choose another pattern, you would want it to be either quite a bit larger or smaller. Depending on the other colors in your bedroom, you'd also be safe in choosing white, brown, beige or gray (unless that is aqua) solids as your duvet color. You could also choose a cover that has two or three of those colors in it. You have quite a few choices, depending on what you like. tibbrix, that first cover is gorgeous.

This one also comes in a blue/grey color.

http://www.potterybarn.com/products/theo-ensemble-brownstone/?pkey=cduvet-covers-shams%7Cpatterned-duvet-covers-shams&&cduvet-covers-shams|patterned-duvet-covers-shams

This may be a better photo. Thanks again for the comments.

I think it's going to be very tricky to mix another pattern with your draperies. If you hit on the right thing it could be smashing; otherwise it's going to be easy for it to look "off". You could us any of those colors in solids, even mix them with a duvet cover in one of the colors and sheets and pillow cases of another. If you want some more pattern/texture, maybe something like one of these?

Ikea. Solid blue/grey, cream, linen, tone on tone. I have lots of duvet covers and switch them out. There's also a cool striped one that has all your curtain colors and is very soft.

Solid linen in one of these colors. Expensive, yes; but the feeling is heavenly. And it holds like nothing else. You can always add a throw or a pillow when you want more pattern or different texture (or two throws and two pillows-I usually keep two throws on my bed, until late June because So Cal is so freezing. Then I keep just one))

I have several duvets, and I usually highly prefer pattern and color; but the simple linen one is the best of them all.

Look at West Elm. They have some nice ones that come in solid colors. This one comes in a gray: http://www.westelm.com/products/organic-cotton-pin-tuck-duvet-cover-and-shams-b340/?pkey=cduvet-covers

My DD recently purchased this duvet cover. The pattern is pretty subtle, so it may work with your drapery panels.

http://www.target.com/p/tile-duvet-and-sham-set-the-industrial-shop/-/A-46767498#prodSlot=medium_1_11

I have the first duvet cover tibbrix posted (Pottery Barn McKenna). After seeing your pic of the drapery panels in your room, I believe this duvet cover would be too much pattern.

Someone posted this company in another recent discussion. I love that the opening is closer to the middle which makes inserting the duver so much easier. For me, I wish the white one came with more choices than just the grey trim, though. However, several of these might work for you.