In my editing classes, we discussed how the color of ink affects the person reading the paper. Red is thought to be very strict (think: teacher's grading essays). Too often black blends in with the printout. On the other hand, blue is a bit too casual. So it's suggested to go with softer but more visible color (think purple, pink, etc.) It kind of "humanizes" the technicality.

But, that's not why I use pink. I just like the color!

(BTW, gel pens are my absolute favorite!)