Gild pure gold, paint the lily
The full phrase comes from Shakespeare’s play “King John.” It actually goes “to gild pure gold, paint the lily.” And though the meaning hasn’t really changed, it sure makes a lot more sence. To cover pure gold with more gold would obviously be decadent. And lillies are already colorful, so they wouldn’t require painting. Though the phrase works as an idiom in its truncated form, we much prefer the poetry of the full phrase.