There are two ways to add another yarn. You could do it in the same direction, or in the opposite direction.

This tutorial will show you how to add yarn in the same direction.

Step 1: With the RS facing you, pick up stitches on the piece, from LEFT to RIGHT. Pick up once for every stitch.

Since you’ll be continuing in the same direction, you’ll want to have the same amount of stitches.

I don’t care which way the stitches are picked up at this point. If they’re the wrong way, I’ll correct that in Step 2.

Step 2: Pick up the new yarn, and knit those stitches from RIGHT to LEFT (in the regular way). Take care to make sure you work all the stitches correctly (this is an issue if you picked up any stitches so they’re laying incorrectly on the LN).

And here’s what it looks like on the WS:

And here’s what it looks like after you’ve worked a few rows:

A couple things to consider

1. Make sure that the colors you plan to use together won’t run when they’re laundered. I’ve heard some success with Color Catchers but I wouldn’t rely on them.
2. Choose where to weave in your ends so that colors with a lot of contrast don’t show areas where a dark color is woven in. That means, in our example above, I would make sure that the white and teal weren’t woven into each other. Of course, different styles of weaving in ends can product other effects. The least risky way is to only weave into to same color.