I explained a little bit about inks used for letterpress in a previous blog post. Now, I'm going to go a little deeper in to ink mixing
Letterpress ink can be rubber or oil based. I use inks from Southern Ink Co which combines both. It dries quickly on paper, yet can stay on the press overnight if I don't have time to clean right away.
If I need to print a color that's not one of the Pantone base colors, I use the Pantone guide. This is the "universal" color language. Each color comes with a formula. I weigh each ink color on a scale and mix it prior to applying it to the press.
Using a Pantone formula guide helps ensure you get the exact color you want. If you say "light purple," I might mix something that looks more grey to you but more purple to me.
In short, the sky's the limit when it comes to creating ink colors for letterpress!