I can visualize a quilt done in shades of purple and shades of orange - but my guess is because your customer said only white if adding a third color that she may not be happy with anything except a solid purple and solid orange and solid white (if necessary). You could do a triple Irish Chain using white for the background, orange for the center of the chain and purple for the outsides. Concur with Cowpie2 that a pinwheel pattern would also work well, as would someone's earlier suggestion of a DNP. Guess what pattern you use will depend on how big she wants it, how soon she wants it, and how much time you want to spend on making it. Be interesting to see what you come up with.