I generally opt for a neutral duvet or comforter (Just like Stacy and Clinton say on What Not To Wear, neutral does NOT have to mean white. Neutral can be gray, metallic, khaki...). My reasoning behind a neutral duvet with patterned sheets is that sheets can be easier and cheaper to change than a duvet or comforter. You can generally find high quality, cute sheets pretty much anywhere! I like to fold the duvet down at the foot of the bed and let the fun patterned sheets do their thang! Don't get me wrong, I love pattern and color and am not afraid of a cute patterned duvet or comforter, BUT this is another option that has served me well. Also, remember to put the top sheet on your bed "inside out" if you want the pattern to show when you fold the sheet back.
Some fun neutral duvet covers
Have fun mixing and matching different styles and textures.
Happy middle of the week!