Well it's nice to experiment. If u want to try them try them I wouldn't worry about what people say especially guys, if it were up to them girls wounds wear makeup at all. I think when done right it looks great, like using it as a pop of oil or like the second pic. I usually just wear a thin line un top similar to the first pic. Like I said if u wanna wear it wear it don't worry about what anyone else thinks as long as you like it. :)
You can use any color you want as long as it's black. I didn't like it when girlfriend tried this, and I didn't like it when the girls in my class tried it. Stick to basic black sweetheart; just forget about brown, blue (girlfriend used this), green, and purple eyeliners. I promise you black is all you need.
I think they can look really pretty, but it really depends on the person's face and eyes. Odd comparison, but it's kind of like how some people can pull off certain piercings while others would look pretty bad with the same ones. All about how your face is suited for the eyeliner.
I don't wear eyeliner, personally. Having said that, I think blue looks nice with brown eyes (blue is the opposite of orange on the color wheel, which can be likened to brown, and thus is why it can "brighten" or "enhance" the appearance of brown eyes). Experiment with different shades to see what compliments your skin tone, as well.
I like copper and really dark blue and more subtle versions of colored eyeliner if they are done well. Like these... 2.bp.blogspot.com/.../eyeliner+2.jpg or https://encrypted-tbn2. gstatic. com/images? q=tbn:ANd9GcQvzokj04nnp0WnflGY1iC44TekISSbnab-1vo2thYMU6KeURr- I find these types of colors look really good with my blue/grey eyes.
I personally don't really like them, the only colored eyeliner-ish thing I did was the blue mascara, and I didn't really liked that one either. I just think black looks the best, as it is to emphasize your eyes, and if you use colour it just takes the attention away from them.