A "reputable dog breeder" doesn't attempt to pass off colors not recognized by the breed standard as "RARE".
NEWFOUNDLAND COLORS as recognized by the Newfoundland Club of America's breed standard are:

Color is secondary to type, structure, and soundness. Recognized Newfoundlandcolors are black, brown, gray, and white and black.

Solid Colors--Blacks, Browns, and Grays may appear as solid colors or solid colors withwhite at any, some, or all, of the following locations: chin, chest, toes, and tip of tail. Anyamount of white found at these locations is typical and is not penalized. Also typical are a tinge of bronze on a black or gray coat and lighter furnishings on a brown or gray coat.

Landseer--White base coat with black markings. Typically, the head is solid black, or black with white on the muzzle, with or without a blaze. There is a separate black saddle and black on the rump extending onto a white tail.

Markings, on either Solid Colors or Landseers, might deviate considerably from those described and should be penalized only to the extent of the deviation. Clear white or white with minimal ticking is preferred. Beauty of markings should be considered only when comparing dogs of otherwise comparable quality and never at the expense of type, structure and soundness.

Disqualifications-- Any colors or combinations of colors not specifically described are disqualified.

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Reputable breeders breed according to the specifications of the breed standard.