What causes the appearance of the parti color in Schnauzers, despite common assumptions?

The result of a specific set of genes being inherited, often recessive ones present in the gene pool

The striking appearance of the parti pattern is fundamentally a matter of genetics within the existing Schnauzer gene pool, rather than evidence of recent outcrossing. The genes responsible for creating these distinct, multi-colored patches are present in the lineage, often carried recessively. This is analogous to how parents with brown eyes, who both carry the recessive gene for blue eyes, can produce a blue-eyed child. In the Schnauzer, these specific color genes are present but are typically masked or overridden by the genes that produce the recognized colors like Solid Black or Salt and Pepper, leading to an unexpected expression when the correct combination is inherited.