(NOTE:This photo is copyright and may not be used without the permission of the ESSSC.

These three beauties represent the three basic colors in Shelties; merle, sable, and black.
The one on the left is a merle, which happens to be a blue merle. There are also sable merles.
The middle dog is a sable.
On the right is a tri-colored Sheltie (black and white with tan markings.) A black and white Sheltie that has no tan markings is called a bi-black.
All Shelties have some white on them. Shelties also come in color-headed whites and mis-marks. Those dogs have over 50% white on them and are not acceptable in AKC conformation competition. While there is nothing that says they may not compete, the rules on color in the Sheltie standard are as written:

Color
Black, blue merle, and sable (ranging from golden through mahogany); marked with varying amounts of white and/or tan. Faults-- Rustiness in a black or a blue coat. Washed-out or degenerate colors, such as pale sable and faded blue. Self-color in the case of blue merle, that is, without any merling or mottling and generally appearing as a faded or dilute tri-color. Conspicuous white body spots. Specimens with more than 50 percent white shall be so severely penalized as to effectively eliminate them from competition. Disqualification-- Brindle.