I've recently being looking into whether Philip IV of Spain was a redhead or not. I haven't found any quoted references yet, but in his pictures he certainly looks to be one. Not only that, many other members of his immediate family look also quite red-haired.
This is Philip IV himself.
Supposedly he had an illegitimate son with an actress called María Inés Calderón - pictured here with red hair.
His son Charles II also looks distinctly red-haired according to his paintings. Charles II died heir-less and was known as "the Bewitched". In the following picture of him his red hair is so vivid it seems to be deliberately highlighted. The picture is quite bizarre actually, I'd be interested to know what all the symbolism in it means.
Philip IV's daughter, Maria Theresa of Spain, was also very gingerish according to her pictures.
However, her sister, Margaret Theresa of Spain looks like more of a blond.
Philip IV's father, Philip III of Spain also looks distinctly ginger.
And one of Philip IV's preceding rulers in the Spanish Netherlands, Isabel Clara Eugenia, also looks curiously red-haired. Again another red-haired Isabella/Elizabeth ruler.
I also found this picture, depicting her and her look-a-likey dwarf ..very strange! Almost Twin Peaks like.