Can you explain the coat colors in Shelties?

There are "base" colors and then variations on top of the base.

The base colors are sable and black, each with varying amounts of white.

The modifier is merling...

So you can have: Sable/White, Sable Merle/White, Black/Tan/White ("tri"), Black/Tan/White Merled ("blue merle"), Black/White ("bi black"), Blue/White ("bi blue").

For more detailed information, visit the following websites:

http://www.athro.com/sheltie.html

http://www.sparkshire.com/color_genetics.htm