Q:

What is the origin of the term "ginger" for red-headed people?

I'd like to know the etymology of the word "ginger" in reference to red-headed people.
In particular, if "ginger" in this context is related to the plant root used in cooking, I'd like to know how the color red became associated with it. I am only familiar with ginger roots that do not possess any red coloring.
A quick google search yields vague and somewhat conflicting results. An authoritative answer with references would be appreciated.
Edit: Although an answer has pointed out a distinction between the term "ginger" and "red-head" in terms of physical features, there is certainly a tendency to see the term applied to people with hair that is any shade of red. I would be interested to trace the usage history of how the word ginger has been applied in that more broad sense.

A:

The OED online has this listed as definition 5.B of "ginger":

B. adj.1 dial.
Of hair: Having the colour of ginger. Of a person: Sandy-haired. Of a cock: Having red plumage.

a1825   R. Forby Vocab. E. Anglia (1830) ,   Ginger, of a pale red colour, particularly applied to hair.
1834   T. Medwin Angler in Wales I. 35,   I perceive a fine red or ginger game-cock in the yard.
1886   R. Holland Gloss. Words County of Chester,   Ginger, sandy-haired. ‘He's a bit ginger.’
1897   Daily News 10 Sept. 2/6,   Complexion and hair brown, moustache ginger.

So, as indicated by "having the color of ginger," someone at some point was quite convinced that the color of ginger was indeed red.

A:

I am only familiar with ginger roots that do not possess any red coloring.

Contrary to many cooks' experience, ginger root (Zingiber officinale) is not the only ginger plant. The Red Ginger plant of Malaysia, Alpinia purpurata, is a brilliant red:

Circumstantially, it was in the late 18th century and the 19th century, during which ginger came to mean “red-haired” or “red-plumed” according to the OED citation in Amanda's answer, that the British occupied parts of Malaysia (among other countries), and started coming into regular contact with the Red Ginger.

A:

It simply comes from the colour of ginger. Its skin isn't really plain brown when cleaned up, but more of a yellow-brown-red mix of varying proportions.

Also, ginger and redhead are technically different as they refer to different shades of red. Ginger is more a brownish red/orange. Then there's auburn! Here are two random links outlining the difference. The reason why ginger is used by some to refer to all red-haired people is probably because of the negative connotations of the word in the UK; guilt by association!

