TheOddOneOut
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Buff Brahmas are a thing, as are black Brahmas. They may not be showable, but they definitely exist. As do White, Partridge, Blue…Of course! Standard Brahmas come in three central colors, Light (white with black) Buff, (Buff with black), and Darks (are grey-ish brown with penciling, black marks).
Your Light (White) Brahma looks correct, but the others are not. (So those are where your mistakes are)
So no, they aren’t necessarily different breeds or mixes.