(g hard). Gay as the king's candle. A French phrase, alluding to an ancient custom observed on the 6th of January, called the “Eve or Vigil of the Kings,” when a candle of divers colours was burnt. The expression is used to denote a woman who is more showily dressed than is consistent with good taste.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894