I assume ground pepper is made by drying peppers and grinding them up. But aren’t most peppers green or red? If so, why is ground pepper black?
Peppercorns are the seed from the pepper plant, not ground-up or dried red or green peppers. (The pepper plant isn’t something that grows in your garden; it’s a tree about 20-30 feet tall.)
Sorry, hit enter too soon.
Slight correction: I believe peppercorns are more properly called “berries” than “seeds.”
See missbunny’s answer, then see here for more details.
Wrong kind of pepper. Black pepper is made by frinding up peppercorns, which are actually dried, immature berries.
To expand a little on missbunny . It’s my understanding that the entire pepper seed is ground up to make the pepper we commonly buy in grocery stores. It is the larger and darker seed casing that gives it the black color. The actual seed, and where we get most of the flavor, is much lighter in color. I think you can buy “white” pepper which is much hotter.
Due to my religious viewing of Good Eats, hosted by Alton Brown, I can tell you this: what the general population calls “peppers” (such as bell peppers, jalapeno peppers, and habanero peppers), are actually chiles. So, therein lies the confusion.
A few more tidbits:
When you’re thinking of bell peppers, you’re thinking of fruits in the genus Capsicum.
When you’re thinking of the black powdery stuff, you’re thinking of the berry from the genus Piper.
Haven’t you ever seen whole peppercorns at the supermarket or used a pepper mill? If not, take a look next time – that’s where ground pepper comes from. You should try freshly ground black pepper if you get the chance – it’s much more pungent and fruity than the preground variety.
Also, peppercorns come in four basic varieties: black, white, green, and red/rose.
I just read somewhere (in a link I can’t find now) that black peppercorns are black because they are dropped into boiling water. They are picked red or pink, and the hot water turns them black. I think the white ones have the outer casing removed. I didn’t think white pepper was any hotter but I could be mistaken.
From my (and Q.E.D.'s) link (actually, from the white peppercorn link at the bottom of that one…bolding mine)
Actually, the red peppercorns are a different species. CiteCite here.
And, yes, the white ones do have the black casing removed, but are also more “mature” (left on the vine longer), than the black version.
Likewise, the green peppercorn you can find in many gourmet shops is a pickled version of the black peppercorn. And has been noted before, I always try to use freshly ground pepper (i.e. right then using a pepper grinder), as the flavor of the pre-ground stuff does not last long, especially the finely ground stuff you find in those little canisters at most restaurants.
Just to clarify any misunderstanding, the “pink pepper” you’ll find commercially (usually in a mix with black, white and green peppercorns) is not the same thing. The “pink pepper” there is from various species in the genus Schinus, and according to this page, has a very mild and not at all pepper-like flavour.
(Another plug for the whole site. The spice pages are under “Hobbies” halfway down.)
(And I see on preview that Level3Navigator beat me to my main point, but the page I linked is prettier. So there! 
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I’m talking about red peppercorns, which usually come in a pickling solution or oil, just like green peppercorns. Those certainly are of the genus Piper. They’re rather difficult to find, but they’re basically ripened green peppercorns.
Peppercorns: black, white, green and pink.
Yes, but those pink peppercorns are not true peppercorns, i.e. from the genus Piper.
I’m talking about the ones in the middle of this page.
As a fellow religious viewer of the same, I can tell you this: chiles are merely a type of berry!