Eyeliner, by itself or with eyeshadow, is a great way to accent your eyes. Most people stick to brown and black, and rightly so, because these two colors work well with any eye color and it's a basic color that can be used for a laid-back look or for a more glamorous look. Just like eyeshadow, there are tons of color to play with.
This color is great for almost all eye colors. Blue-eyed people, since purple is next to blue on the spectrum, I'd use discretion about using purple because the wrong shade will draw more attention to the mismatch with your blue eyes. Definitely lean towards dark purple if you want to try it out.
I love Urban Decay's 24/7 Glide-On Eye Pencil in Rockstar. It's dark purple, with a little sparkle. I like to pair this with blue mascara (see my post "True Blue Baby I Love You") and/or a blue-purple eyeshadow like Urban Decay's Eyeshadow in Asphyxia (lavender with blue sheen). There are so many shades of purple, so be sure to pick a shade that accents your eyes and goes well with your skin tone.
I think this color looks good on everyone, even those with blue eyes! It has a tendency to appear black in some lighting, so the blue is quite subtle. Because it's not as dark as black, it's easy to tone up or down. Again, if you want to bring out more of the blue in the eyeshadow, pair it with blue mascara. Check out Laura Mercier Kohl Pencil in Black Navy and Make Up Forever Aqua Eyes in Iridescent Navy.
Charcoal and Brown
In addition to my staple black eyeliner, I really like these two colors, too. It's great for everyday work makeup. I like to pair my brown eyeliner with light beige eyeshadow.
With so many options with eyeliner color and effects (sparkle, glitter, iridescent, etc), I suggest testing the color on your skin first, and if you're unsure about which shade of a certain color, test a whole bunch of them in one area (on your hand) and compare. Once you've found one that pairs well with your skin tone, try it on your upper eyelid.