I didn't say the Lower Kingdom was a Mixture of Negroes, I said they WERE Negroes, or at least Ethiopians (who are QUITE distinct from West Africans in Appearance and physical structure).
The Combined Kingdom DID become a mixture and under the Nubian Kings quite definitely more Ethiopian-like.
Ramsses with red hair
There may have been something in the mummification materials that turned hair a reddish- brown. This is seen also on the mummy of Kind Ramsses II. I think it would probably only show on the mummies with white or grey hair though.
I just did a Google search and found the information you posted about Ramses having dyed hair. The only problem is you left out the part where the French scientists examined the roots and confirmed they showed he did in fact have red hair, although it would have been white when he died.