Tricolor is a black body coat with tan and white markings. Blue merle can range from powder blue to gun-metal gray, with varying amounts of black, white and tan markings. In effect, where the dilute gene touches the black, it dilutes to a shade of gray. Bi-black is also a black coated dog, but without the tan markings. Bi-blue is the same as a blue merle, but without the tan markings.
Sable merle is a sable dog with the dilute merle gene applied over it.
Sable merle is considered by some to be a controversial color as it is not directly addressed by the AKC Standard for Shetland Sheepdogs. There are no health issues associated with the sable merle, just as there are none with the blue merle.