Candle
Heat, nerve and blood — the colour every tradition agrees about, roughly.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Red is the steadiest entry in the colour tables. Agrippa puts fiery red under Mars, alongside iron and the martial temperament; Culpeper's herbal runs on the same planetary logic, filing the hot and stinging plants in the same place. Four centuries later the modern lists still say courage, vitality, desire and defence. Where correspondences agree this well across sources, it is usually because the association is doing something obvious — red is blood and fire, and nobody had to be taught that.
The work follows. Practitioners use red for nerve before a difficult thing, for physical vigour, for desire, and for the more assertive end of protection — not warding a threshold quietly, but standing in a doorway.
It is a hot colour and it behaves like one. Red workings have a habit of producing more than was asked for, and experienced practitioners tend to cut them with something cooler or burn them for short fixed periods. White will substitute; orange gives a gentler version of the same push.
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)
Mars in Agrippa takes iron, red, and everything hot, sharp and quick to burn. He is notably wary of it — martial workings in his account tend to overshoot, and he advises tempering them with a gentler planet rather than running them alone.
Carried forward
Common in contemporary practice
Burned in short fixed sessions rather than left to go out on their own. The common advice is to decide beforehand how long a red candle stays lit, because deciding afterwards rarely happens.
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.