I'm not a pro but this is what I would do.

Pick a color from the wallpaper. Either the leaf (greenish) or the flower (lavender?). If you have a scrap of it, bring it to a paint shop to match it exactly OR go either darker or lighter.

Here's a quick match up on Photoshop.

Unless you're really attached to that curtain, a white curtain or the other color you don't pick for the wall would work too (e.g., lavender wall, light green curtain or vice versa).

Have fun!