Things You'll Need
When knitting, you can work in any combination of colors. Fair Isle knitting involves carrying two or more colors along the back of your work as you knit in order to make small splashes of color in a knitted garment. Intarsia knitting involves working with two or more colors of yarn separately in order to have larger blocks or sections of color. Whether you want to make complicated patterns or simple stripes, you can cast on with more than one color of yarn.
Cast on the first color of yarn for the specific number of stitches your pattern calls for. For example, let's say you want to knit a scarf that alternates red and half blue in a vertical stripe pattern. You would start with the red yarn and cast on approximately 10 stitches using any casting-on method you prefer.
Drop the first yarn at the back of the needle and let it hang down.
Pick up the second yarn and cast on the number of stitches your pattern calls for. Let that yarn hang down in the back of the needle when you finish casting on.
Pick up a third yarn and cast on the number of stitches your pattern calls for. If you want to use your first color again, you will need a new ball or length of yarn in that color; you have to have a separate working strand of yarn for every color in your pattern row, rather than carrying one ball of yarn back and forth.
If you are only going to need a few yards of yarn, it is easier to cut those yarns off of the balls of yarn before beginning to knit. This way you can pull them out of a tangle more easily.
Be careful when turning your work, because having more than one ball of yarn means you have to watch out for tangles.