Candle
Hidden things and high office — a colour whose grandeur started as a matter of price.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Purple's association with power has a very practical root. Tyrian purple was extracted from sea snails in absurd quantities — thousands of molluscs for a small amount of dye — and was consequently so expensive that its use was restricted by law in several places. Purple meant rank because purple meant money, for well over a thousand years before it meant anything about the psychic.
The modern reading builds on that: authority, mastery, and the study of hidden things. Divination, dream work, and the deeper end of learning are the usual uses, and Jupiter is the most common planetary assignment, with Neptune appearing in twentieth-century lists. It is a good colour for work you intend to do properly over months rather than perform once.
Practitioners often reserve purple for the parts of practice they take most seriously, which may be the whole of the correspondence — a colour that once could not be wasted still gets used carefully. White substitutes; blue serves for the calmer end of the same work.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Heinrich Cornelius Agrippa, Three Books of Occult Philosophy (1533)
Agrippa treats colour as a way of addressing a planet directly, purple standing among the royal shades. His lamps and candles are instruments rather than decoration, and he is specific that the material, the hour and the colour are meant to agree with one another.
Carried forward
Common in contemporary practice
Kept for divination and dream work only, and not used for daily candles. The reason usually given is that a colour used for everything stops signalling anything — which is a claim about the practitioner, not the wax.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.