1. A period of excellent performance, where nearly everything seems to go right, work properly, and contrasting with a more general lower level of performance.
The England team hit a purple patch just after half-time, where they scored 3 goals in 10 minutes, but in the end they were lucky to escape with a 3-3 draw.
2. An ornate or elaborate section of a written work, a patch of purple prose.
Purple prose is a term of literary criticism used to describe passages, or sometimes entire literary works, written in prose so extravagant, ornate, or flowery as to break the flow and draw attention to itself. Purple prose is sensually evocative beyond the requirements of its context. It also refers to writing that employs certain rhetorical effects such as exaggerated sentiment or pathos in an attempt to manipulate a reader's response.
When it is limited to certain passages, they may be termed purple patches or purple passages; these are often noted as standing out from the rest of the work.
The term purple patch is also used in a more general, and more unequivocally positive, sense to refer to a period of outstanding achievement. This usage is particularly common in sporting contexts in some countries; for example, a footballer who had scored in six successive games might be said to be "enjoying a purple patch".
The term "purple prose" is derived from a reference by the Roman poet Horace (Quintus Horatius Flaccus, 65–8 BC) who wrote in his Ars Poetica (lines 14-21):
Inceptis grauibus plerumque et magna professis
purpureus, late qui splendeat, unus et alter
adsuitur pannus, cum lucus et ara Dianae
et properantis aquae per amoenos ambitus agros
aut flumen Rhenum aut pluuius describitur arcus;
sed nunc non erat his locus. Et fortasse cupressum
scis simulare; quid hoc, si fractis enatat exspes
nauibus, aere dato qui pingitur?
Your opening shows great promise, and yet flashy
purple patches; as when describing
a sacred grove, or the altar of Diana,
or a stream meandering through fields,
or the river Rhine, or a rainbow;
but this was not the place for them. If you can realistically render
a cypress tree, would you include one when commissioned to paint
a sailor in the midst of a shipwreck?
Purpureus meant lustrous or dazzling in Horace's Latin.
source: wikipedia, wiktionary