I'm in love with these Faber-Castell PITT artist pens (markers). I first ran into them at Artist &Craftsman Supply in Cambridge, a wonderful little store I'm glad to call my favorite.
The brush tip gives a greater control over the type of line you want and the Indian ink is not completely opaque so you can layer it up where you want darker tones.
This tonal layering technique works just as well with hue variation. Leaving gaps between the colors in some places and overlapping in others allows for a satisfying blending of color.
Here are a couple portraits done with my beloved markers:
Self Portrait, Summer 2012
Cheryl, Summer 2012