I’m no expert, but to my understanding, the acidity of the bog only breaks down one type of hair melanin. Eumelanin comes in brown and black and is responsible for blonde, brown, black, and gray colors in hair. Phaeomelanin causes pink/reddish hues. They differ chemically enough that eumelanin gets stripped down while phaeomelanin remains relatively intact – hence why the bog mummies have orange hair rather than pure white. I imagine a natural redhead would remain a redhead as a bog mummy, though their hair might lighten or intensify in redness as any eumelanin was broken down. That’s the best of my understanding.
maybe he’s born with it
maybe it’s 2,000+ years submerged in an acidic German peat bog