Is there historical truth to Cao Zhang's beard being Yellow? Any story to go along with that?
Actually, yeah. Cao Zhang’s Sanguo Zhi contains a quote from Cao Cao in which he refers to Cao Zhang as 黃鬚兒, “My yellow-bearded son.”
A passage from the Weilue, appended to Cao Zhang’s biography, also says 彰鬚黃, “[Cao Zhang] was known for his yellow beard.”
黃 in this case should probably interpreted as meaning light colored hair, rather than distinctly yellow or blond as we know it, and there might be some metaphor here that I lack the cultural context to understand. But as far as the literal text goes, Cao Zhang is described as having a yellow beard.
So, there’s no particular story behind it. Historical records - for no particular reason - just describe him as such.