As a former diamond buyer, I've been asked this question many times, worded in different ways: "Which of the four-C's is least important when buying a diamond?" My answer to this question? Color.
When color-grading a diamond, the stone is held upside down on a white surface, to determine what degree of body color is visible. This process makes color very obvious, much more so than it would ever be to the naked eye. When the diamond is right-side-up, with the crown visible to the eye, color is much less apparent. In addition, once placed against the skin (whether it be on a ring, pendant or earring) your skin color will be seen a bit through the stone, making any diamond appear slightly less than perfect.
My advice to those in search of that perfect diamond is this: If you need to save some cash, sacrifice color rather than size, cut or clarity. You could save thousands, and the diamond will still be gorgeous. That being said, J or K is as low a color grade as you should go, without seeing the stone in person.