i just re-read your question and realized that the video i linked you to might not address those things.... i'm not sure i understand the first part of your question, though, the keeping yarn in the back when you're on the back side.... does the fair isle not occur on the back? if so, you'd have to be purling, which means the yarn is in the back when you're working and it should stay there...
as for using a color you haven't used, you have a couple of options. you can "carry" it up the rows along the edges , so that way it is available at the beginning of the row you need it on or you can keep it active throughout by stranding it, which is shown in the videos.
Sean (click for my blog)
and/or visit me on Ravelry (boyforpele13)
Share Your Life With a Deaf Dog!
Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure. We ask ourselves, Who am I to be brilliant, gorgeous, talented, fabulous? Actually, who are you not to be? We were born to make manifest the glory of God that is within us. And as we let our own light shine, we unconsciously give other people permission to do the same. - Marianne Williamson