The darker shades of eyeliners are designed to define your eyes, while the lighter shades of eyeliners are designed to brighten your eyes. Use a darker shade of eyeliner on your eyelids and a lighter shade on your waterline for maximum effect. So, choose accordingly and use accordingly.
If an eyeliner is misbehaving with you by being overly soft, you should draw a thick line using this eyeliner at the back of your hand. Follow it by using an angled liner brush to pick up the color and beautify your eyes.