First, a useful distinction – consider a stop sign:
This is a sign, which communicates meaning via convention and the word ‘Stop’: there is nothing about a red octagon that, in itself, means stop;
The color red, on the other hand, is used in the stop sign symbolically – unlike the word ‘stop’ and the octagon, red has meaning that precedes its use in a road sign. That meaning comes from universal human experiences, not from any conscious choice. Further, the meaning of the color red is at a deeper level than words – saying red represents blood and divinity and burning power, while true, does not mean that anyone *gave* those meanings to red.
A red octagon is a very good choice for a stop sign – easy to recognize, attention-grabbing color, plus the octagon shape is rare in nature, so the sign is unlikely to be mistaken for a natural object – a good thing, given its purpose. It is a convention built on common human experience. A green circle would not be nearly as good a stop sign, because the color and shape conflict with the intended meaning.
Christian iconography is like the stop sign, only much richer. It starts with colors and other natural objects redolent with meaning and uses them to communicate something deeper. It is a subset of the more general presence of symbols in Christianity, many if not most of which come straight out of Scripture: water, bread and wine, the cross, lambs, fish, and so on – it is not only or even primarily that these symbols are used in representational art – they dominate the poetry, the prayers, and the rituals of the Church, and have since the time of the Apostles, who lived the New Testament and were steeped in the Old.
These same symbols are used for icons of the saints, plus a new set of signs and symbols proper to the saints themselves. For example, St. Lawrence, an early deacon and martyr, is always portrayed with a grill, because he was famously grilled to death. St. Catherine is portrayed with a wheel, the means of her martyrdom. Less grim to modern sensibilities, St. Dominic is often portrayed accompanied by a Dalmatian dog holding a flaming torch in its mouth – because the Dominicans wore black and white – the colors of the dog – and Latin phrase ‘Domine Canes’ means ‘dogs of the Lord’ – so the Dominicans were the Lord’s Dogs, spreading the light of the Gospels – thus the torch. Puns and other jokes are embraced by the iconographers, as long as they get the point across.
If the iconographers are doing their job well, a single set of conventions, with a small number of variations, will develop for each traditional subject – no one had to make this happen, apart from the artists efforts to get the image right. The icon is right when the viewers understand it and see the deeper connections clearly. At that point, changing what has now become ‘traditional’ will become merely confusing – it will look wrong. For example, if you portrayed the Blessed Mother dressed in bright green, no one would believe it. The Blessed Mother wears a red or white with gold trim gown, or just maybe a gold gown – but that’s pretty much it. To the viewer, is you portrayed her dressed otherwise, you’d be trying to tell them something – it couldn’t just be ‘because I like green better’. If she’s wearing a cloak, it will be blue – almost always midnight blue – or black. You could just maybe get away with brown, but you’d have some ‘splainin’ to do.
To sum up: Iconographical symbols are natural object and colors with deep, pre-logical meaning. Signs are more conventional carries of meaning, which may or may not use symbols to help bolster the clarity of their message. Christian iconography uses both symbols and signs to get across deep, pre-verbal messages and the specific identity of the persons and events portrayed. Each subject commonly portrayed – a saint, a biblical story, a martyr’s death – will have quickly developed a set of symbols and signs which become conventional because they strike everybody as ‘correct’. An artist who deviates from these conventional representations risks losing his audience.