Seen in the Late Middle Ages, "White" armor was a form of armor without a surcoat. Around 1420, the use of surcoats began to disappear (in lieu of uncovered or Maced Plating). The term "White" was not meant to denote a color, but lack of style or trimmings (plain). However by the mid to late 16th Century, the term came to be used to identify highly polished armor (very bright upon contact with sunlight or "gleaming").
Conversely, the use of "Black" armor meant plating that was treated or burnished dark - hence "Black". Examples of both White and Black armor can be found in most European museums.