E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
White Czar (The).
Strictly speaking means the Czar of Muscovy; the King of Muscovy was called the White King from the white robes which he wore. The King of Poland was called the Black King.
Sunt qui principem Moscoviæ Album Regem nuncupant. Ego quidem causam diligenter quierbam, cur regis albi nomine appellartur, cum nemo principum Moscoviæ eo titulo antea [I van III.] esset usus Credo autem ut Persam nunc propter rubea tegumenta capitis Kissilpassa (i.e. rubeum caput) vocant; ita reges Moscoviæ propter alba tegumenta Albos Reges appellari.Sigismund.
The marriage of the Czarevitch with the Princess Alex of Hesse (2 syl.), will impress the Oriental mind with the expectation that the Empress of India and the White Czar will henceforth labour to avoid the mischief of disagreement.The Standard, April 21st, 1894.