Red: Red letter days are so called because in old church calenders feast days were written in red ink. In American banking ledgers of the early twentieth century debts were written in red ink and credits in black. An Italian debtor is said to be in the green. Either way one is in the soup.
Orange: Oranges were originally a fruit and didn't become a colour until 1542. For this reason in the Nun's Priest's Tale Chauntecleer the Cockerel describes his nightmare about a fox thus:
Me mette [dreamed] how that I romed up and doun
Withinne our yeerd, wheer as I saugh a beest
Was lyk an hound, and wolde han maad areest
Upon my body, and wolde han had me deed.
His colour was bitwixe yelow and reed
And tipped was his tayl and both his eeris
He then goes on to defy laxatives.
Yellow: Has always stood for cowardice, it was believed that a lack of blood made the liver yellow. It's odd that nobody seems to like to mention this colour. I can think of no yellow political party and even yellow hair is called blond/blonde (the only adjective in English that agrees with its noun) or golden. The yellow jersey of the Tour De France is the one exception that I can think of.
As Bob Dylan wrote in Tombstone Blues:
Well, John the Baptist after torturing a thief
Looks up at his hero the Commander-in-Chief
Saying, “Tell me great hero, but please make it brief
Is there a hole for me to get sick in?”
The Commander-in-Chief answers him while chasing a fly
Saying, “Death to all those who would whimper and cry”
And dropping a barbell he points to the sky
Saying, “The sun’s not yellow it’s chicken”
Green: The sixth stanza of Anderw Marvell's The Garden runs thuslyly:
Meanwhile the mind, from pleasure less,
Withdraws into its happiness
The mind, that ocean where each kind,
Does straight its own resemblance find.
Yet it creates, transcending these,
Far other worlds and other seas;
Annihilating all that's made
To a green thought in a green shade.
The second couplet, by the way, is based on the idea of contemporary biology that each land animal had an equivalent in the sea. However, even at the time Thomas Browne (the Inky Fool's favourite essayist) was inclined to disbelief:
THAT all Animals of the Land, are in their kind in the Sea, although received as a principle, is a tenent very questionable, and will admit of restraint. For some in the Sea are not to be matcht by any enquiry at Land, and hold those shapes which terrestrious forms approach not; as may be observed in the Moon fish, or Orthragoriscus, the several sorts of Raia's, Torpedo's, Oysters, and many more, and some there are in the Land which were never maintained to be in the Sea, as Panthers, Hyæna's, Camels, Sheep, Molls, and others, which carry no name in Ichthyology,
Blue: Blue murder is not a rather saucy form of homicide: it is a translation of the French exclamation mort bleu, which is a corruption of mort dieu, meaning death of God. Blue murders are likely to be carried out by blue-rinse blue stockings.
Indigo: is one beyond blue, it is therefore a very sad colour. Blues musicians have nothing on their indigo colleagues:
For there's nobody who cares about me
I'm just a poor fool that's bluer than blue can be
Is therefore a lyric from Duke Ellington's Mood Indigo*.
"Roses are red
Violets are blue":
That's what the song said,
But it cannot be true.
"I have seen roses
Damasked red and white"
And he's always right.
And a blind man could see
That violets are violet
Such truths are inviolate.
And after that we drift into the invisible parts of the electromagnetic spectrum, which is mostly white noise.
Violet in colour, I think you'll agree.
*Duke Ellington wrote neither the melody, the title nor the lyric; but the matter is so vexed that I shan't go into it here.