there are a number of small motifs that only involve 1 color, books like Nicky Epstien's Knitting on the Edge (and Over the edge, and Beyond the edge, and Book (Vol) 3 of the Vogue stitchinary, (color)
have a selection..
you can do a heart, or a clover, or bow (Nicky has several Pink bows of various styles and sizes!)
some patterns, (multi color ones) also lend themselves to being 1 color, (they end up looking like a silhouette).
stitch collections, (treasuries) often have several.
if you don't have any, check out what is available in the local library, or make up a design of your own!