I've been experimenting with Micron Black and Gelly Roll White pens on my gray sketchbook. I love the effect of the shading. I use the white gel pen to add highlights where the tangle highlight normally would have been left. I normally shade in "deep" areas, on both sides which leaves a "highlight" or "shiny" look to the curves. A white gel pen can highlight those areas even more by accenting that area. For some awesome new tangles, check out Ellen Wolters on YouTube!
Here's some more examples of tangle with white highlights... I think it adds a nice touch.
This I called the "Cat's Meow!"
I hope you enjoy these new techniques as much as I do. Try your tangles with different pens, inks, and on different colored papers. I think you'll be surprised!