For my current project it took me forever to find the right shade of grey for my borders.
Usually I have trouble finding something in the right color AND style. For example, there's a few Japanese style prints...something with sprays of cherry blossoms, but I can never seem to find something in black with the pink cherry blossoms. Most stuff I've seen have backgrounds in pink or some sort of pastel. I think when I was working on my Zelda quilted stained glass project (before I lost all the friggin files and now have to start over) I had trouble finding marbled or watercolor fabrics in certain colors. Had even more trouble matching some solids for my Mario quilt (again, before I lost everything for it and have to start all over).