If you want to softly define your eyes, then it's very important to pick an eyeliner color that is not black. Black tends to dramatically draw attention to the eye area. It looks harsh on most skin types and should be used only at night when you want to play up your eyes with a smoky, intense look. Even navy and charcoal are sometimes too deep for day wear. For daytime you can't go wrong with having three different eyeliner colors in your makeup bag. A plum, khaki and a soft brown or copper will enhance any eye color, especially if it's applied very close to the lash line for the most natural definition.
It's important to pick colored eyeliner hues that complement your eyes, not ones that match them or clash with them. For example, if you have blue eyes, don't select a turquoise liner. Instead, opt for plum, gold, or orange liner to bring out the blue in your eyes rather than wash it out and create a faded look.