Generally speaking, any brown eyeliner gives off a softer and more mellow look than black, according to All Day Chic. Black is dramatic and can look a bit harsh, especially in the daytime. On the other hand, brown is classy, neat, and more subdued. Further, brown also looks great on any skin tone and works well for any eye color, too. 

Eyeliner is meant to enhance your eyes. You want people to see their beauty first, not eyeliner coming at them from a mile away. Though brown is naturally understated, it's this subtlety that makes it so captivating. Brown eyeliner looks great when paired with a light brown matte eyeshadow, but you can also add a brush or two of shimmery light brown to make it really pop. 

Despite the common notion that only black eyeliner outlines the eyes perfectly, you can still give them a good deal of definition using brown eyeliner (via Maybelline).