At some point, you will have to join a new ball of yarn to your work in progress. Whenever possible join the new yarn at the beginning or end of a row. With garments or items with seams, the yarn ends are best woven into the seam. The yarn ends are more secure in a seam, than woven into the body of the piece. Joining yarn elsewhere in a row is unavoidable when color knitting, or knitting in the round.
The yarn required to complete a row is approximately 3 to 4 times the width of the piece. If you have this much yarn left, work the row. If you don’t, start a new ball of yarn.
Some knitters drop the old yarn and begin knitting with the new yarn, tying the yarn tails into a knot, to deal with later. This method can be sloppy and yarn ends can easily work their way to the right side, through wearing and washing. It also results in a loose edge that needs to be fixed. Some books mention splicing plied yarns, but this is tedious, unless you are skilled at hand spinning. My preferred method is to overlap the old and new yarns at the side edge, knitting the first two stitches with both yarns.
Steps: Overlap the end of the old yarn with the beginning of the new yarn, and work the first stitch with the two strands of yarn. Drop the old yarn end, pick up the tail end of the new yarn and the new ball yarn, and work the second stitch, leaving the short ends on the wrong side. Drop the short end of the new ball of yarn and continue across the row with the new yarn. Leave the short ends of yarn on the wrong side. Pull snuggly on the two short ends. When coming back across the row remember to work the double strands at the edge as single stitches. When your piece is completed, these yarn ends are woven into the seam or wrong side. This method works well for most yarn weights, except very bulky.
How to Join Yarn When Color Knitting
Method 1: You can simply use the overlap method described above, and weave the ends in with a tapestry needle on the wrong side when the piece is finished. This is best with a few colors, or large blocks of color. When working with many colors, weaving in yarn ends with a needle is very time consuming. To save time and effort, work the yarn ends while joining the new ball of yarn as described in Method 2.
Method 2: In this method, you are simply weaving the short ends of the new and old yarns along the back of work, by laying the ends over top the right needle, and either knit or purl with working yarn, until completely knitted in. Steps for the knit side: Cut the old yarn, leaving a long tail, about 6 inches. Knit the first 2 stitches with new yarn. Lay the short ends over the right needle and * knit the next stitch with the new yarn, bringing it under woven yarn. When you knit the next stitch, the woven yarn is already under the stitch. Repeat from * until short ends are completely woven on wrong side.
Steps for the purl side: Leave a tail as described for the knit side. Purl the first 2 stitches with new yarn. * Insert needle purlwise into the next stitch, laying the short ends of both old and new colors over top the needle, and purl the stitch under the short ends. Leave the short ends hanging, and purl the next stitch over them. Repeat from * until the short ends are completely woven on the wrong side. (The image below shows the purl side or wrong side, so you can see how the short ends look woven in).
Note: I still like to use the overlap method for the first two stitches, instead of just knitting the 2 stitches with the new yarn.
Give the overlap method a try, and I promise you will be pleased with the results.