To start a new color, just start using it, and later you can weave in the end, as shown in the link for joining new yarn. (Basic Techniques/More page.)
If you are doing fair isle on a flat piece of knitting, then knitting just one row with one color presents a problem: the other color will be on the wrong end to knit with, if you want to use it next. Solutions:
-Knit two rows with that color, instead of one.
-Use a cable needle so you can slide to the other end (or just transfer your stitches so you're at the other end of the needle), then you can do a single row with just that other color and you'll end up with both colors on the same side and can use the two together again.
-Cut the yarn of the old color and rejoin it at the new working end.
-Do what I think is called "weaving," where you secure the other color behind the stitches of the single color, so it's carried invisibly across the back of the row.
Is that it? I think I covered all the solutions! Oh yes, and, of course if you knit it "in the round," you don't have to worry about any of these things, because after one row with just one color, you will end up back where you started and the other color will be there waiting for you.
Hope that helps! (And is not information overload for you! LOL!)
KnittingHelp Queen Bee
"There is a vitality, a life force, an energy, a quickening that is translated through you into action, and because there is only one of you in all of time, this expression is unique. And if you block it, it will never exist through any other medium and it will be lost. The world will not have it. It is not your business to determine how good it is nor how valuable nor how it compares with other expressions. It is your business to keep it yours clearly and directly, to keep the channel open. You do not even have to believe in yourself or your work. You have to keep yourself open and aware to the urges that motivate you. Keep the channel open."
— Martha Graham