Being a drafsman, yes we still use technical pens, they make good coffee stir sticks
, I have used India Ink extensively to highlight panel lines, welds, and other molded in features on ships and planes.
This type of ink thins very light and that is why we prefer it, and you and adjust the tones with more coats. The nice thing about India ink is that once it dries, it cannot be affected by water and most solvents.
Now you can also get artists inks, which come in all your monotone colors. These are red, yelloy, blue, white and black. I mix white and black to adjust my "grey scale" of the wash or a little yellow and white for a slight "aged" look. Again, these inks allow you more versatility with adjustng the quality of your wash than you would get from using model enamels of acrylics.
Brand names I have used are Higgins, Black Cat, and Koh-I-Noor for the black India inks and brands than make multiple colors are Winsor & Newton, Dr. Ph. Martin, and Gamlin.
Cost of these are around $2 to $4 a bottle. Hobby Lobby might carry these but I have found them to be hit and miss. For artist materials, I tend to get them here.
http://www.dickblick.com/
Scott