Looking in the EXIF data of the image we see:

Exposure = 1/125s F4
Lens = 70mm maximum aperture f2.8
Camera = Canon 10D
Shooting date = 13th September 2006

This image was probably done on the (at the time) top-of-the-line Canon 28-70 f2.8 L lens. I think that the purity of the colour starts from there - I've noticed that my top-of-the-line Nikon lenses sometimes have something extra in the colours compared with the others.

The use of a telephoto lens with a wide aperture has given limited depth of field, as Adina says. The lighting is excellent but then it's just a matter of getting the model to stand in the right patch of sunlight, with light reflected off the ground (sand?) to fill in the shadows.

I'm almost sure the image has been retouched. Her skin has been smoothed to remove the flaws. The whites of her eyes have been brightened to give her a more piercing look - too much so on the left eye in shadow, it doesn't look natural to me.

Oh - and the camera seems to have a hot pixel visible in her right eye (left as we look at it) just below the pupil..