Found it!

While I was cleaning up the table for dinner last night, I found my scrap of bright yellow paper mixed in with the pile of fabric I hadn't cut yet. It turns out I'm almost done cutting. I'm going to make a last quick scan of the sewing room to see if I can find any more promising drab fabrics lurking there, then I may just drop everything else and see how many little postage stamp squares I can piece before my enthusiasm fades.