E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
characterises authority and power, not sanctity. The colour indicates the character of the person so invested:The nimbus of the Trinity is gold; of angels, apostles, and the Virgin Mary, either red or white; of ordinary saints, violet; of Judas, black, of Satan, some very dark colour. The form is generally a circle or half-circle, but that of Deity is often triangular
The nimbus was used by heathen nations long before painters introduced it into sacred pictures of saints, the Trinity, and the Virgin Mary Proserpine was represented with a nimbus; the Roman emperors were also decorated in the same manner, because they were divi.