The International Quilt Study Center & Museum has a show about indigo quilts up now through June 2, 2013.
Indigo is a difficult dye---difficult to apply and difficult to understand. It's extremely durable so dyers and printers devised many ways to get different effects.
Once you see this show and read the gallery guide and the labels you will have a good understanding of those different printing and dyeing technologies.
The guide and the exhibit explain the technology, but if you can't go---
Curators Marin Hanson and Nao Nomura detail it on this webpage:
You can download the gallery guide as well as see all the quilts and read the labels.
Like many things though, you don't have to understand how indigo works to love it.
Oak Leaf 1844 (date inscribed)
IQSC Patricia Cox collection
Delectable Mountains, estimated date 1890-1910
IQSC Dillow collection