[Yes, I intend to work my way through the spectrum, and possibly out to white, black, and brown, depending on how much fun I'm having. This first 'article' is a lot longer than I'd intended, because red has the most history. The bulk of the info is about historic dyes used to produce it - if that stuff bores you, try reading the intro and the exciting conclusion. I've put the names of specific colorants in bold, so you can skim through and read the bits you're interested in. As you know, I find all this stuff fascinating, and it's my blog, neener neener.]
Red is the oldest color, in terms of human use and how long people have been purposely coloring things red. It was easy to do, because iron oxide that occurs naturally in soil will 'stick' to nearly anything (often whether you want it to or not). This is reflected in the history of the actual WORD red. In English it can be traced, linguistically, all the way back to proto Indo-European, which is as far back as linguistics can go. None of the other color names can be traced back this far. (For instance, orange, the youngest color name I know of, can only be traced back to the 1500s. Before that it was yellow-red, and didn't have a specific name.) In Russian, 'krasnaya' means both red and beautiful. Which gives another insight into just how old the word is in that language, and how far back the color was available.
As mentioned above, the oldest of the red pigments was red soil, caused by iron oxide (a.k.a. rust). Ochre is the most common name for this, though the term Indian Red refers to a specific brownish-red soil originally mined in India. The use of this colorant goes all the way back to the stone age; archeologists have discovered a forty-thousand year old ochre mine in Africa, and it's probably the most common colorant in cave paintings, after charcoal (black) and white (chalk). It was used back then to color leather clothing, also. As a paint, it is still used; depending on the exact chemical content and the binders used with it, the exact color can vary widely. As a dye, ochre isn't used commercially any more (at least not beyond novelty levels), but I'm sure there are native peoples out there still using it. In Hawaii, the high iron content of the soil makes it red; any hiking turns your shoes red. A company called "Red Dirt" dyes shirts and other clothing with the soil as a novelty tourist gimmick. The color is not completely colorfast, and the shirts fade much like indigo-dyed denim.
Plant dyes were most likely up next. The most common ones still in use are brazilwood (South America), madder (EurAsia), and amaranth (assorted species worldwide). Brazilwood produces a red-orange dye that's colorfast, though alum must be used to 'fix' the dye. Brazilin is the modern name of the dye that's extracted from the bark. Madder, and it's modern chemical relative, alizarin, are also still in use. Madder's been found in Egyptian tombs, Pompeii, and all over Asia. Madder is the reason so many of the flags of older countries contain red; it was an easy colorfast color (fairly colorfast) to produce. Amaranth was used by Native Americans as a dye, but these days is a food dye - red 2 in the US and E123 in Europe.
The next dye discovered, probably, was crimson. It comes from insects. More specifically, squished insect 'blood'. In South America, the Aztec and Maya grew cochineal insects on cacti; in Europe, the kermes insect was grown on oak trees. Once cochineal was introduced to Europe, kermes production fell out of favor; much easier to get the colonies to provide it. It has been used to dye fabrics for probably a thousand years in South America (at least), and longer in Europe, to dye fibers of all kinds, both protein and cellulose. The exact shade of red varies by how the bugs are 'processed' (killed). It's one of the more colorfast and chemically stable of the natural dyes, and is considered safe for food products. In the US, it is food additive E120, found in everything from eye makeup to Cherry Coke. Yum yum. Scientists are still arguing about whether it causes potentially fatal allergic reactions, but the evidence looks pretty condemning to me. Oh, and as a food additive, it's not even remotely kosher.
I'm sure the discoveries of these dyes were not as neatly lined up as I have them, but hey, I've gotta organize this somehow.
Another 'family' of red colorants hark back to ochre, namely, different minerals and elements. None were used as dyes that I know of, but were common painting pigments at one time. Red lead is still in use in some places. It's an oxide of lead that's made by heating it (all those lead fumes - wonderfully healthy). Just over the last ten or twenty years, industries have been phasing it out in favor of less toxic alternatives (for instance, Waterford - the lead crystal company - used to use red lead paint as guidelines for cutting the crystal, and long-time employees occasionally wound up with cancer or other problems). Cinnabar, China red, and vermillion are all historic names for variants of mercury sulfide, and as such, are toxic as hell and react to many other chemicals, like the stuff in the paint beside it on the canvas. This, along with ochre, goes back to prehistory; exactly how far, we aren't sure. You can still buy this stuff, the real deal and not a synthetic imitation, but you will pay through the nose for it (a hundred bucks, plus, for a small tube of paint). I can't find any instances of fabric being dyed with cinnabar. I'm not sure it's possible, and suspect that if it is, wearing the fabric next to your skin would kill you (red lead as a dye might do that too, now that I think of it).
In 1856, William Henry Perkin synthesized a variation of red - namely magenta, though he called it mauve - from coal tar. After that the history of dyes gets really boring and reads like a chemistry manual.
Red clothing (which is what we're interested in, as knitters, I presume) is flattering to nearly all skin types. At different times worldwide, red clothing has been confined to the elite. Because the color has been relatively easy to reproduce (if you don't believe me, wait until we get to purple and black), at least for dyers, it's been popular since the stone age. From an official, color-theory point of view, red's complimentary color is green. It blends best with orange and purple, and contrasts most with yellow, blue, and green.
There you go. More than you could possibly want to know about red.