Ombre means shade or shadow in french. An ombre mani is when you go from light to dark on each nail or across the hand. All I did was use the original purple color, Mira, for the pinky nail. Then I progressively added white polish to the purple to make it lighter for the other fingers. A more detailed description is on my blog...
You should give it a try...all you need is a saturated color that you like and white polish to mix in with it.