Figuring out how to ink my pieces has been the most irritating grailquest of my life. So unless you have a specific painting in mind, the answer is “it varies”.
1 2 3 were “inked” using pencil—-usually either a mechanical pencil with medium soft lead, or a wood pencil of various leads. Note a piece might be colored with watercolors OR photoshop.
1 2 3 were inked either with platinum carbon ink, which is very waterproof, or with sakura ink, which is not but is cheap and is fine for work that will be colored digitally. This would be the ink I recommend to anyone who wants to use watercolor on top of their inks. I used a few of the smallest Windsor and Newton series seven brushes in this case.
1 2 3 were “inked” simply with dark watercolors with the aforementioned brushes. I find a dark indigo mixed with burnt umber works well, but other colors may be used. I also am experimenting with water-soluble colored pencils, sharpened to a very fine point. Faber Castell makes a very beautiful pencil of this type. This method gives the softest look, I find.
Lastly, since acquiring a Cintiq I’m trying to ink with it. I use Kyle Webster’s brushes for photoshop exclusively for digital coloring and inking.
But! I’m always trying new things, so if you ask me again in a year the answer may be totally different. The key is figuring out what feels good, what marks feel honest and unique to your hand, and working to master them so they are no longer entirely accidental but at least partly in your control. I think every artist comes to their mark only after a mountain of shitty drawings, but once they find it they recognize it as true.