Changing colors to make striped or patterned knits is usually the first thing beginning knitters want to learn after the knit and purl stitch. This technique for changing colors can be used for stockinette and garter stitch. You can also use the same technique for switching between more than two colors!
Make a Slip Knot
Cast On Stitches
A row of purl stitches will make stockinette stitch.
Switch to Contrast Color
If the stripes are thin enough, you may want to carry the yarn up the side.