The smoky eye look can be pulled off in several ways, but the most common is to use three colors: a highlight, a midtone and a contour. The highlight is the lightest of the three shades, and is applied high up on the lid beneath the eyebrow. The midtone is the middle shade, and it's used on the middle of the lid and in the crease. The contour is the darkest of the three shades, and it gets applied closest to the lash line.
All three shades should be in the same color family -- so, for instance, you'll want a dark purple as a contour, a medium purple as a midtone and a very light purple or neutral color as the highlight. Many eyeshadow kits come with these three shades all in one, so if it's your first try at the smoky eye look, a kit might come in handy.
So what color family should you choose? Remember that what makes the smoky eye look is that it's dark and deep -- so sticking to bolder shades will create the desired look. Your contour color should be strong: brown, black or gray is the norm, but a deep plum, a dark navy or even a forest green can rejuvenate the look.
You can match your eyeshadow to your outfit, like Scarlett Johansson did with magenta eyes to match a magenta dress at the Oscars. Or, go in a completely different direction to bring the focus up to your face, like the bright purple eyeshadow Rihanna wore with her red dress at a Gucci event to benefit UNICEF.
Now that you've got your colors picked out, let's arrange them in just the right way. Next up: layering and blending.