What causes the less common Blue/Rust and Fawn/Rust coat colors in the breed standard?

Answer

A specific recessive gene that reduces the pigment intensity

The Blue coat appears as a diluted black, and the Fawn coat is a diluted red or brown; these dilute colors are genetically caused by a specific recessive gene that reduces pigment intensity.

What causes the less common Blue/Rust and Fawn/Rust coat colors in the breed standard?

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