If you are trying to knit with two colors, one at a time, you basically just "take turns" with each color. When you are knitting, the yarns are held in the back of your work, as always. Purl is in the front. When the pattern calls for, say, red, you knit with the red and carry the, say, white across the back (or front for purl). When the pattern calls for white, you use the white and carry the red. The problem comes in if there is a large gap between the change in colors. Then you have to trap the color being carried across the back. Any books that carry directions on fair isle knitting usually have techniques. You have to maintain an even tension, always sliding the yarn down your right needle so it doesn't bunch up.
If you are talking intarsia, or putting a picture on your knitting, you'd also be best to get a book on it, because there are techniques that can make the difference between a bunchy, bumpy, holey pic or a smoothly incorporated one.
I, for one, LOVE to knit with color, both kinds, so would be happy to share my experience. It's just that there's so much to cover. :XX: