The subtly textured print of these curtains by west elm would coordinate with your gray walls and outstanding quilt. I would use the yellow sparingly as accessories. http://www.westelm.com/products/cotton-canvas-printed-crosshatch-window-panel-t090/?pkey=cwindow-panels-curtains-shades

You could even go with a subtle striped pattern with the white/offwhite as the background, then choose one of the colours in the bedding for the stripe (yellow if you want!)

I'm just a little worried that a curtain with any pattern will be a little busy with the patterns in the bedding. But I do love the west elm panels you sent. I may have to check those out.

Sometimes a little bit of pattern is ok as long as you're in the same colour families and you pay attention to scale. I actually love mixing patterns. Here's an options that would look great!
http://www.landofnod.com/not-a-peep-curtain-panels/f9836?t=6&a=185 (this is a kids curtain, but you can definitely use it)

Originally my thought was an ivory curtain with a faint, thin, yellow stripe...something very subtle.