I've
not used inks a whole lot but this is some of the things I've found out about
inks.
Some colors of ink can discolor when left in direct
sunlight.
Inks can be used to add shading. They can add detail and bring out detail on a figure. You can also use them to detail certain parts of the figure such as uniform insignias, or other things you may want to show on a uniform. I started using them while I was painting some knights and found them very helpful in putting designs on cloaks and horse blankets.
You can pick inks up at craft stores such as Michael's or art shops. For the most part I've found mostly primary colors and I have found some good uses for them.
I picked up a couple of the cheap ink dip pens. They may call them Calligraphy pens. They are good for using with the ink and detailing shield designs. I also sometimes use the pens with paint. They don't have cartridges and you just dip them in the paint or ink like you would a brush. You do have to work with this because the pen doesn't hold ink or paint like a brush. You'll also want to make sure you get the finest point you can. Also make sure you clean the pen up when you finish with it.
Now you can use a brush with your ink. You want to use a 10/0 or 15/0 or any of the fine point or liner brushes. These will allow you to do any detail inking you wish to do. Some of the bigger brushes like 5/0, 4/0 and 3/0 will allow you to cover the bigger areas.
When I use inks I tend to use them much as a wash. They tend to add some shading and detail to a figure. You'll want to work with them till you get the affect you want. For this part, I use a brush with the ink and brush it onto the figure. I have played around with coating the figure with clear coat and letting it dry then using the ink as a wash. Basically, I brush it on. The gloss coat tends to keep the ink or wash from blending in with the paint already on the figure and the ink tends to flow into the groove or indentations and dry. You don't have to do the coating before the ink but sometimes you may find this to give you a very nice affect especially with armored figures.