There are many different ways to join a new length of yarn to the one that is already being used. Like most knitting techniques, the method you choose depends on your personal preference and the specific project situation. Experiment with each and pick what works best for you!
Slip Knot Join
This join introduces the second color of yarn by securing it to the first color of yarn with a slip knot. The yarn ends are worked back into the fabric later on using duplicate stitch.
Knit to the point in your work where you need to start a new color.
Make a slip knot in the new yarn and thread the old yarn through the slip knot.
Slide the slip knot up the old yarn until it meets the back of the fabric. Tighten it lightly.
Begin knitting with the new yarn. The slip knot anchor will hold the end in place so that your initial stitches remain even.
Knit several rows past your join. Go back to the temporary slip knot and undo it.
Twist the yarn ends around one another once to avoid leaving a hole in the work.
Work each yarn end in back along its matching strand using duplicate stitch.
Leave the yarn ends so that they fall into a matching color area. This will help to prevent them from showing on the other side of the fabric.