How a yarn stripes itself
The dyer applies color to the yarn in lengths. A yard of red, a yard of cream, a yard of blue, repeating: when you knit, each length becomes a band of rows, and the fabric stripes with no color changes at all. The width of the band depends on one thing — how many stitches are in a round — because a yard of yarn makes more rows in a narrow tube than a wide one.
That is the whole trick, and it has three variations.
Short stripes
The classic sock yarn. Colors change every few yards in a fixed sequence, designed so that on a 60–70-stitch sock each color makes a stripe of three to six rounds. West Yorkshire Spinners Signature 4ply is the clearest example in stock; Berroco Sesame and Wizard do the same at worsted and chunky weight, and Juniper Moon Zooey Stripes and Cumulus Rainbow bring it to summer cottons.
Knit at sock width they stripe as drawn. On a 40-stitch wristwarmer the stripes are wider; on a 200-stitch shawl each color is less than a row, and the stripes dissolve into a fleck. A blanket from short-stripe sock yarn is a tweed, not a striped blanket, which is sometimes exactly what you want.
Long gradients
Schoppel's Zauberballs, Reggae Ombre, Amano Riti, HiKoo Concentric and Edition 3 are dyed in runs of many yards, so one color fades slowly into the next across a whole hat or a third of a shawl. The width of the piece still matters, but less: a gradient that takes an inch of sock to shift takes a few rows of a shawl, and both read as a fade. In Zauberball Crazy and Starke 6 two plies each carry their own gradient, so the fabric marls as well as fades. These are the yarns for one-skein shawls, cowls and hats where the color is the whole design, and the Zauberball guide sorts the family.
Noro's color runs
Noro dyes the fiber before spinning, in long planned sequences, so Kureyon, Silk Garden, Ito and Tsubame move through a series of distinct shades with soft edges between them. They stripe like a gradient yarn in a sock and like a long-stripe yarn in a scarf — a Silk Garden scarf is the classic use, each color a few inches wide — and in a sweater the runs are long enough for the stripes to stay bold. The Noro guide has the lines.
Matching two socks
Two socks from one ball of a short-stripe yarn will match if you start the second at the same point in the sequence as the first: find the color where the first cuff began and wind off to it. A 100 g ball holds a pair with a little over, so the loss is affordable. With a gradient yarn the two socks will not match unless you split the ball by weight and knit one from each end, and most knitters let them differ — it is the usual look.
Stripes and stitch patterns
Stripes fight with cables and lace: the eye cannot read both. Stockinette, garter, plain ribbing and simple slip-stitch patterns let the yarn do the work. One trick worth knowing is to knit a short-stripe yarn in a slipped-stitch or mosaic pattern, which breaks the stripes into a tweedy check that looks far harder than it is.
Hand-dyed is not self-striping
A hand-dyed variegated skein — Malabrigo Arroyo, Dream in Color Jilly — has colors applied in short, irregular dabs. It does not stripe; in a narrow tube it can pool into blotches, and in a wider piece it speckles. That is a different effect and a different guide; the hand-dyed shelf is where those live.