Color Theory
Some of you will just be able to blend colors and fibers without a second thought. You can grab 4 to 5 balls of yarn or fiber and just know that they'll go together. Some never need to take a color theory class... you'll look at color charts and away you go. Others need a color theory class to make it click somewhere along the way. Once you learn the tricks to color blending, you can soar.

No matter which category you fall in, it's never a bad idea to sit down with a color wheel or some color charts and play for a while. I remember in color theory class in college, we had massive color pages where we had to mix paints and make teeny tiny squares. Gradients from light to dark, mixing the primaries, shading and lightening the primaries, shading and lightening the mixes...and so on. It was tedious at the time but it does leave you with the feeling that you really get what color mixing is about. Blending and making colors goes way beyond the primary school version of yellow and red make orange.

This exercise is not necessarily about blending red and yellow to make orange, but it does help to know a little more about color and how color works together when choosing fiber colors for a sweater. First find yourself a color wheel.

many thanks to The Artist's Toolkit for the image!
Check out their great site for more information on color theory.

The Lingo
The terms below are a simple way to start thinking and talking about colors as you begin your blending adventures.

Primary colors are the colors that we all grew up with. Red, yellow and blue. The basic colors that we use to mix and base other colors on.

Analogous colors are colors that are adjacent to each other on the color wheel. If you want something to blend well, you want to stick to within a couple of movements to the left or right. For example, red will blend well with the purple ranges until it gets too blue or with the orange ranges until it gets too yellow. That's not to say that yellow and red or yellow and blue don't work well together, but they give a different feel.
Complementary Colors are colors that are opposite each other on the color wheel. Pairs like blue and orange, red and green, purple and yellow. They complement each other in the way that help each other visually pop.
Primary, analogous, and complementary are the main color terms that we will be using. There are many more terms in color theory but we're sticking to the basics today.. I'm going to add just two more terms that specifically speak well to hand dyed fiber.
Neutrals go with everything: grey, white, brown, black. We use them in the same way that we use all the other colors.
Triads are worthy of mention because they holds up so well with handpainted fiber and yarn. Triads are 3 colors that you can see as a true triangle on your color wheel. Purple, green and orange for example.
If we were to take a look at handpainted fibers and yarns, we'd see that the ones that are pleasing (even if we don't personally like a color) fall within some of these rules. They're analogous, complementary, triadic, neutral or a combination. Maybe they're analogous with a pop hit from a complementary color. Or maybe there are two sets of analogous colors that are complementary. Pick out the patterns that you see. How do you want your blending to go? More analogous? Complementary? Some of both?

Then we have some yarns/fibers that use the color wheel in a unique way. We can use them, but need to know how. Let's take a look at some handpaints and how to put them together.

Let's start with handspun. Or rather, let's just start with the fibers to make your handspun. I like to combine fibers that are similar. Merino behaves much differently than Romney does. This wouldn't necessarily stop me from putting them together, but it might make me try to find an alternative for one or the other. The Less is More sweater used all medium grade wools.

For this example, I've chosen medium grade wool top. If you have the space, lay out a bunch of wools that you like. If you really have the space, lay out ALL your fibers and have a look. Pick two to four fibers that you really want in your sweater. Don't fall in love yet... you might not get to keep them all.

Pick one that jumps out at you and let's call that color A. Do any of your other favorites have similar colors to A? That one gets put next in the line up and we'll call it B. Is there another one of your favorites that has anything similar to B? keep choosing in the same pattern until you've chosen your 4 or 5 colorways.

Line them up. Squint. See how they go together. Don't see them blending well? Braid them (it bunches the colors and will help simulate what it's potential is as a handspun) and then see if you can pick out similar colors. Does A flow to B and then B flow to C and C flow to D?

The process with commercial or handspun yarn is the same. I still like to work with similar fibers again. Though in this case, more important than fiber content is the weight of the yarn. You really want to keep them very close in gauge. Less is More used a DK weight yarn, so we'll pull out all of those and take a gander. Just as we did for the fibers, pull your favorite colorways then start matching, one color at a time until you have a line up of 4 yarns. A and D do not need to go together -- they just need to match neighbors B and C.

It is helpful to make a cardboard swatch. Wrap a piece of cardboard with your yarns in the order you want to use them in your sweater, slightly overlapping where the colorways will change.

All in all this may take some time and you may need to acquire a new yarn or a new fiber to fill in a color space. The more you do this exercise, the better and quicker your eye will become at making the matches.

This is a chance to step out of the box a little bit. Purple is not one of my favorite colors, but in a working set of handpainted yarns or fibers that has a touch of purple, it works for me. When mixing it with other colors, the purple is less obvious to me, thereby making it more enjoyable and wearable.

Play! Make your cardboard wrapped swatches. Sit on the floor with your yarn or fiber all around you. Have fun and make something wonderfully colorful.