Before the invention of powered flying machines, humans could only move through the air using balloons carried by winds. Now we can travel more or less anywhere at speed, and we are no longer confined to Earth, so the mystique of flying is much less that it was hundreds of years ago. Nevertheless, we are fascinated by birds because they evolved powered flight using their own bodies and there has always been a rich mythology attached to birds and the places that they can reach. Using our imagination, we borrowed one of their major attributes – wings – and used these in our depiction of flying creatures like angels that are portrayed as humans with bird wings that enable them to fly between Earth and Heaven. This image has persisted for centuries and probably originated in statues of Greek goddesses, such as Nike, that have a clothed human form, with wings in addition to arms (when they are not broken off... see above).
In paintings, most angels are shown having predominantly white wings, an example being Guercino's St Sebastian succoured by angels (below, upper), although there are exceptions to this convention, as seen in works by Fra Angelico (below, second) and Veronese (below, third). Bright colour was a common feature of places of Christian worship in the first half of the last millennium, so it is not surprising that religious paintings are similarly brightly coloured. White wings, however, have another symbolic role. They remind us of doves, released as symbols of peace and reconciliation, and long portrayed in paintings as representing the Holy Spirit, as in the Double Trinity of Murillo (below, bottom).
If white is good, then black is evil in western Culture and birds like crows have a quite different reputation to that enjoyed by doves. They are known from ancient folklore as bringers of bad luck and even death , and they are potent symbols of evil in paintings, such as those by Giovanni Bellini (below, upper) and Mantegna (below, middle). Even though these illustrations are details from larger works, the prominence, and isolation, of the birds conveys a powerful message. Crows add to the forbidding tone of Van Gogh's Wheatfield with Crows (below, bottom), painted shortly before he died from infection following a self-inflicted gunshot wound. Although the overall colours of the painting convey gloom, imagine how the work would look without the crows and then think whether the atmosphere would be more, or less, menacing.
From mythology then, we conclude that white, or brightly coloured, wings are features of good angels, while evil angels, or angels of death, have black wings. In contemporary culture, we extend this metaphor by using capes; items of clothing designed originally to provide freedom of movement. Note that Superman (below) has a red cape that is fastened at the shoulders, just like angel wings. We know that Superman can fly, and that he came from another world, and we also know that flight was achieved through superhuman powers, just like those of angels. The cape is a symbol of Superman's ability to fly, just as bird wings are for angels, and it separates him from the merely human.
Although they do not appear in the same fantasy, Darth Vader of Star Wars (below) represents the antithesis to Superman. We see him as an imposing figure in black with a black cape held at the shoulders, just like that of Superman. The colour reminds us of evil and the visual imagery works before we ever know of Darth Vader's intentions (by the way, the "baddies" often wore black hats in early Western movies).
It is fun to think of Superman as a good angel (with some messianic qualities) and Darth Vader as an evil angel, but how odd they would look if earlier conventions had been followed and they were given bird wings of an appropriate colour instead of capes. Can you imagine it?
 Steve Roud (2003) The Penguin Guide to the Superstitions of Britain and Ireland. London, Penguin Books.