Some shades of eyeliner work better on blue eyes than brown eyes and vice versa. That's why Allure has some information for you. According to their site, here's the best eyeliner color to fit your eye color:
"Contrasting shades bring out the eye color, so copper and gold make blue eyes appear even bluer."
"Red complements green on the color wheel, so pick shades with reddish undertones."
"You can't go wrong with earthy browns, but "amber and midnight-blue tones emphasize the light flecks in brown eyes."
"Emerald and gold bring out the green tones in hazel eyes."
What eyeliner color do you usually use?