Toonian Smooth Collies in Croatia have a very interesting webpage that talks about the genetics of the collie colours and the coat patterns in an interesting and easy to understand way.

If you are really interested in this work track down the book:

The Collie: A Veterinary Reference for the Professional Breeder -  by Dr Sharon Vanderlip, 1984.
Hardcover: 296 pages
Publisher: Biotechnical Veterinary (January 1, 1984)
Language: English
ISBN-10: 096127560X
ISBN-13: 978-096127560

While it was published some time ago the section on colour inheritance and the Irish colour pattern distribution in collies is still the best in print today.