Let's talk about color! Specifically, about how you can incorporate two or more colors into your knitting. In this post, we'll just briefly discuss the methods that are out there, and then subsequently, I'll get more into the specifics of the more involved methods.
The very simplest way to incorporate multiple colors into your work is with horizontal stripes, where each entire row is worked in a single color. The execution of this method is easy: just put down the old color, and pick up the new one, and continue knitting. However, while the method is simple, the effects can be complex, and offer endless scope for explorations. Fat stripes, thin ones, a mix of each? A mathematical sequence for choosing the width, such as the Fibonacci sequence where each number is the sum of the two previous numbers? Random numbers chosen with a die? A repeating color sequence played against a repeating number sequence of a different length, as in Stephanie Pearl-McPhee's charming tulip socks? How many colors, and how do they play against each other? A simple method -- not necessarily simple results. [Editted 9/1/06: Here's a nifty tool to play with in this arena -- the Random Stripe Generator!]
This is probably a good point to digress just a bit, and talk about how to bring a new yarn color in. There are a number of ways to do this, and I'm certainly not going to exhaust the possibilities here. My personal favorite is just to start knitting with it. I hold the tails of the old and new yarn together in the hand opposite the one I hold my working yarn in, and just start knitting with the new yarn. After 3-4 stitches, I let go of the tails, but when I come back to those first few stitches on the next round or row, there's a tendency for them to loosen up, so I grab the tails again while I work into those stitches, and then after that everything stays nicely in place. When I weave the rest of my tails, I check to make sure there's no funky looseness around the change point, and then weave these tails in as well. If that method feels too insecure to you, I'd suggest loosely tying a knot, but then untying it before weaving the tails. Knots tend to make their way to the front of the work, over time. There are some exceptions to this, most of which involve felting, but as a general rule, no knots should be left in the finished work.
Also, let's touch briefly on dealing with the unused color. There are two options here: either break the yarn and join it again when you need it next, or carry the color up the edge of the work, by loosely drawing the strand up (optionally, it may be twisted around one or more other strands) to the next row where it will be used. For thin stripes, the latter is often desirable, as breaking the yarn each time leads to a large number of ends to weave in, as long as it can be done unobtrusively. For large stripes, it's less desirable, because the long strand up the side that results can interfere with the stretch of the knitting, and may catch on things or become obtrusive. My personal rule is about an inch; if a yarn will be used again within about an inch, and the edge won't be exposed -- it's going to be in a seam, or there will be a buttonband or other border, or the item is worked in the round -- then I'll carry, and otherwise I'll break. There's definitely an element of personal choice in here, however; some people will tolerate longer floats up the side, and others won't tolerate this much.
A slightly more complicated variant of the simple stripe is slipped-stitch colorwork. In this type of colorwork, each row is knit in a single color, but some stitches are slipped instead of being worked; this brings a stitch of the prior color up into the middle of the current color. There are some limitations to this type of colorwork; in particular, it really doesn't work well to slip a stitch up more than about two rows, because serious vertical distortion will start to occur. Because of this limitation, this method lends itself well to very geometric designs with the color in small segments, and because these designs frequently resemble mosaic tilework, this method is often called "mosaic knitting". Barbara Walker has done a great deal of spectacularly beautiful work with this technique, and her book Mosaic Knitting has recently been released in a new edition with additional designs; it's just lovely, and highly recommended. For some other resources, let me suggest this article from Kristi Porter, and this article with accompanying tutorial from Wendy Wonnacot. One particularly instructive thing to note is that Kristi and Wendy show a couple of different ways in which mosaic patterns can be charted; Wendy shows a traditional chart, where every row is charted, and also shows the use of garter stitch to accent the colorwork, while Kristi shows a form of chart that's specific to mosaic knitting, where the chart includes a single line for every two rows of knitting, which is more compact to write and present, but is generally limited to a stockinette ground.
Another technique which can be used for colorwork is double knitting, where two surfaces are worked at the same time on the same needles. This technique is not necessarily used for colorwork -- it can be used in a single color to create a double thickness of fabric, or to knit a tube on straight needles -- but it can be used for colorwork effects when the two surfaces are knit in different colors, with reversible designs produced by switching the colors. The double thickness of fabric that's an inherent part of this technique can be both a limitation and an advantage, since it does produce a nice clean surface with no wrong side; it's particularly handy for scarves, and a nice example of this is Exchequered by Alice Bell for Knitty. Fully explaining double knitting is beyond the scope of this particular post; I'm merely going to note that it can be used for colorwork, and refer you to tutorials such as these from StitchDiva.
You can also fake your colorwork after completing the knitting, with duplicate stitch. Duplicate stitch is essentially embroidery, following the contours of the original knitted stitches. To duplicate a stitch, you will come up at the base of it, where the two legs of the stitch emerge from the stitch below, go behind the two legs of the stitch above, and then go back down where you came up. The limitations of duplicate stitch are that it creates a fabric that's doubly thick, but only where the colorwork is, which can affect the drape and stretch of it, and that the background color can peek through. For small areas of colorwork, however, it is extremely handy. It can also be used to embellish purchased knitwear.
Finally, there are the two methods that most people think of when they hear the word "colorwork" -- intarsia, and stranded methods such as Fair Isle. And no, while Fair Isle is a stranded method, and is often used to mean stranded methods generally, it is not the only stranded method out there, just as every facial tissue is not a Kleenex; when we get to discussing the stranded methods specifically, we'll talk about the distinction. For now, I'm just going to note some generalities about these two classes of colorwork, and then they'll each get their own post.
Intarsia takes its name from the woodworking technique, where small pieces of different woods are fitted precisely together to create intricate designs. In intarsia knitting, the individual color blocks are self-contained, joined to the other colors only at their edges, and a different piece of yarn is used for each block of a color. There are exceptions to this, as when two blocks of a color are separated by a very small portion of another color, and some limited stranding may be employed, but for the most part the blocks are merely linked, and not overlapping. Because of this, intarsia is best suited to large areas of color, as the number of ends may otherwise become unwieldy, but this is more of a practical limit for the convenience of the knitter than an inherent limitation in the technique. There is one significant inherent limitation, however, in that intarsia is best worked flat. It can be done in the round, in a process similar to doing short rows in the round, but this may be more trouble than it's worth. We'll cover this in more detail in the next post, which will be specific to this technique.
Stranded colorwork involves, as the name implies, drawing the unused color or colors in strands across the back of the work. The length of these strands, or floats, provides the major limitation to this type of colorwork: it is undesirable to let these get too long, as they tend to get caught on things and pulled, distorting the work. For that reason, stranded colorwork usually has fairly small areas of color, with any longer areas broken up with flecks of the contrast color. Many knitters have a marked preference for doing stranded colorwork in the round, as this avoids the need to purl, but it is entirely possible to do it flat. Stranded colorwork is also often accompanied by a technique called "steeking" wherein extra stitches are inserted for the armholes and the front and/or neck openings, and then cut open vertically. The two are not inextricably linked -- it is possible to do steeks on non-colorwork knitting, and to do stranded colorwork without steeking -- but they are common companions, so I will touch on steeking as well when I get to the post specific to stranding.