<shrug> I would have thought it was in honor of Phylicia Barnes because her favorite color was/is purple.
$25,000 reward in Phylicia Barnes case - CharlotteObserver.com (http://www.charlotteobserver.com/2011/01/29/2018438/25000-reward-in-phylicia-barnes.html - broken link) but Lawndale isn't close to Monroe.
Hmmm... Purple Ribbons can be used to commemorate all types of things:
Purple Ribbon Products, Awareness Products Online
OK, I'll admit; I don't really know.