Truth
We may never know the absolute truth, but James Hewitt has denied these rumors. He claims his affair with Diana began two years after Harry's birth and that they did not even know each other before. Additionally, red hair runs in Diana's family, the Spencers, which could explain Harry's ginger locks. This means it might simply be a coincidence that Diana had an affair with a red-haired man. News of the World reports that they tested Harry's DNA against James Hewitt's and found no relation. Of course, we can never be certain if that is true.
