purple passage (PUR-puhl PAS-ij) noun, also purple patch, purple prose
1. A brilliant passage in an otherwise dull and uninspiring work.
2. A piece of writing marked by ornate, florid style.
[From Latin pannus purpureus (purple patch), a phrase used by poet Horace in his Ars Poetica (The Art of Poetry) to suggest a patch of royal fabric on an ordinary cloth.]