"And the merchants of the earth shall weep and mourn over her; for no man buyeth their merchandise any more: The merchandise of gold, and silver, and precious stones, and of pearls, and fine linen, and purple, and silk, and scarlet, and all thyine wood, and all manner vessels of ivory, and all manner vessels of most precious wood, and of brass, and iron, and marble," St John Revelation 18: 11-12
A depiction of the Whore of Babylon, referred to in Revelations. Crucially, she is wearing a dress of purple (though it looks red)
"He would wear a tunic made wholly of cloth of gold, or one made of purple, or a Persian one studded with jewels, and at such times he would say that he felt oppressed by the weight of his pleasures."SHA, Life of Elagabalus 2.23
In both cases, purple is held up as a luxury, the first as one that will no longer be wanted by the people of Babylon, the second as a decadent indulgence by the Emperor.
What exactly was purple, and why was it a noun?
In the instances cited above, "purple" refers to Tyrian purple, a dye from Tyre in Asia Minor. It is secreted by a kind of sea snail. As with all things ancient, there's a myth about its discovery: that Hercules' dog chewed some, and his mouth turned purple. It was possibly discovered by the ancient Phoenicians, but there is definite archaeological evidence for it as far back as the 18th century BC.
Here is a 16th century version of the Hercules myth by Peter Paul Rubens
Since ancient times, the "secret" of how to make Tyrian purple has been lost. In 1909 the main compound was identified, but has never been commercially manufactured. An experimental archaeology project, working with incomplete instructions from Pliny the Elder succeeded in dying wool (and their arms) a deep purple, so perhaps we shall see it returned yet.
Purple as a Symbol of Power
Purple has often been considered a symbol of royalty or of high-status. We say people are "Born into the purple", and when he came to power Nero banned anyone but the Emperor for wearing the colour. Alexander the Great, the Selucid Emperors, the Ptolmaic Kings and the Roman and Byzantine Emperors all wore clothes dyed with Tyrian Purple. Mostly this is because it took thousands of mollusks to make even an ounce of the dye making it literally worth more by weight than gold.
In this famous mosaic from the Basilica of San Vitale, Ravenna, Emperor Justinian wears a purple cloak.
But why isn't it still a noun?
It never really was. In the above English uses "Purple" is short for "Tyrian Purple". But you know what? I think it was better the first way.
Edited to Add: Okay, so it is really a noun, courtesy of the comments section.