Have you ever purchased a multi-color yarn that didn't work up like you expected it to? Perhaps the colors on the hank or skein didn't match what you wanted the finished object to look like. Perhaps an unexpected color pattern showed up along the way. When the colors of a yarn group up in particular places, that is called pooling. The color pooling in a finished object can really make or break it. You can have a lot of pooling or a little. The pooling could be patterned or random. There could be spots or stripes.
Here's the most important thing about pooling: It changes based on your stitch pattern. So, while it may be tempting to give away a yarn skein when the colors didn't thrill you the first time, wait! Try again. Another stitch pattern might make all the difference. Pooling can also be affected by the length of your repeat.
For this pooling exploration, I used a discontinued skein of Caron Simply Soft that my friend Vicki gave me, which hopefully represents something you might have laying around in your stash. I worked it up in a number of stitch patterns, trying to maintain around the same width.
In this first picture, I really like the pooling in the smaller stitches. The single crochet on the bottom left and the linen stitch on the top right look good to me. However, I don't really enjoy it in the taller stitches. The shell stitch at the top left looks a little muddy to me, and also has some vertical stripes going on. I also don't think this yarn looks good in the round at all, in either the double or triple crochet. What this tells me is that it could make a nice simple scarf, but I probably wouldn't want to use it for a hat.
I'm not the biggest fan of the pooling in almost any of the samples in this picture. The post ribs, v-stitch, double crochet, and triple crochet all look muddy to me. I think this is because this is a short color changing yarn instead of a long one, so in taller stitches there are lots of colors per stitch instead of just one or two. The knit garter stitch on the bottom right doesn't tickle me, either. However, the striped look of the stockinette above it is nice in a very different way from the single crochet.
There you have it, a small look at pooling. Have fun experimenting to find what looks best to you.