I have tried a lot of different inks. India is good... but it is very black.
I also use a Radidoliner graphics pen. It has a 0.18mm line that is very very fine.
Using a drafting/artist's dipping pen I have grown to like Private Reserve Gray Flannel ink. It is very dark gray, and leaves a very nice dark, dark gray line. It can be watered down and made lighter as well.
I think the Dark Gray ends up looking more realistic.
www.naplak.com/modeling ... a free site for modelers
www.scalehobby.com/forum/index.php ... a nice Modeling Forum