How common is it for the natural color of head hair and pubic hair to differ?
A… friend… of mine has blonde hair on his head, and red pubic hair. Is this uncommon?
Sounds like aftermarket drapes.
It is quite natural for collar and cuffs to be of differing shades. In my experience, the downstairs has tended to be darker than the upstairs. YMMV. 
This is why a blonde who is blond both up and down is called a “True Blonde”.
My beard hair is different colour to my head hair. Which I found rather surprising.
I dunno. My hair is a medium brown (You can see me here), and my pubes are completely black.
I’m darker downstairs that I am upstairs, and my beard is a third shade entirely.
Please tell me what this means, Gabe. 
“The drapes don’t match the rug” is an idiomatic way of saying somebody’s dyed their hair because the hair on their head is a different color from their pubic hair.
Ok, I just didn’t get the word “aftermarket”. What does that mean?
bought later?
aftermarket = apparently non-humorous way of saying “dyed”
Hair backdropped by body will tend to look darker than hair on the head, though not extremely much so. But a friend I went to school with and shared gym-class showers with, back in the day, was a true carrot-top up top but had auburn pubes – and I’m certain he didn’t dye either!
My own case may be interesting, too: my grandfather had dark red hair as a young man – he was nearly totally bald with a white fringe by the time I came along. My own head hair color I inherited from both parents, and it was a drab dull medium brown. But when I let my mustache grow in, it varied over time from auburn to strawberry blonde, definitely not the same color or even a shading from my head hair. (My head hair is now white and my mustache would be grey if I let it grow.)
As a teenager, I still had naturally blonde hair on my head, but my pubic hair and eyebrows have always been a medium brown. My blonde hair darkened to light brown (I assume - I’ve been dyeing it crazy colors for years), while my pubic hair has remained the same. In other words, the rug and drapes can be mismatched without one or the other being aftermarket.
My ex boyfriend’s head hair was VERY blond. His facial hair was a deep red (so it was in his boxers too).
I knew another guy who had brown hair and a red beard. Pretty common from what I understand. Though all the guy’s I know that are two toned like that are of Celtic descent.
Yes and yes.
Aftermarket = parts, etc. purchased after buying something like a car. Aftermarket parts are either replacements or upgrades. Dying your hair would be similar to installing aftermarket parts on your car.
Or The Real Blonde. Funny thing, IIRC, is that the real blonde in that movie was played by Elizabeth Berkley. Doper aficionados of *Showgirls *(I’m told there are one or two) may be surprised at this.
For me, carpet and drapes and the same color, but for some strange reason, if I try to grow a beard, the chin region comes in red, everything else, same dark brown. My grandmother on my mother’s side has red hair, the only one in the family on either side. This is the only explanation I can think of.
My brother was born a strawberry blond, and his hair darkened to “sandy” in his teens, but his beard grows in RED - and there is a distinct line of demarcation somewhere in the middle of his sideburns! His eyebrows are nearly black. I have no knowledge of his, err, carpeting, but I’ve seen him in a pair of shorts only and he has dark hair on his legs and under his arms. Not a single drop of Celt in him, either - he is 100% Italian (although half the family is Northern and the other half is Sicilian.)