The big "scary" danger to collie/sheltie people is the controversy with color-headed whites. The argument is that you can't tell the difference between a color-headed white and a healthy double dilute. So a novice breeder could breed a sable or tri color-headed white to a merle a have a whole litter of double dilutes. The issue here being that the dog they thought was a color headed white is really a dilute. IMO that person shouldn't be breeding anyway because all it takes is a little pedigree research to see that its obviously not a color headed white.
It may just be that I JUST got home from work and I'm so tired I could just pass out on the floor right now, but I'm not sure I'm following this...
Sable and tri color headed whites will look NOTHING like a double merle. Now I could see how a extremly new novice (who shouldn't be breeding anyway) could MAYBE mistake a blue merle color headed white as a double merle, but even then it's a stretch because MOST of your color headed whites are going to have much more coloring to them than most double merles. Even a sable merle double merle I saw a picture of once, was pretty obvious a double merle, based on the lack of color on the head and the washed out color that was there.
Here is kind of an interesting article written for colliesonline a couple years ago
http://www.colliesonline.com/may2009/coat-of-many-colors.php
The sable-merle is also a result of "duh" why didn't you know more about your dog?! As (I think) Romy said perviously sable merle is obvious at birth. It doesn't take a genious to see that if a sable dog has ANY blue in it's eyes it's a sable merle. Also, I'd just assume any sable puppy out of a merle x sable breeding carries the merle gene.
I certainly dont' find this to be true. That is like saying any tri puppy out of a blue x tri breeding carries the merle gene. I have done a few sable to blue merle breedings and actually have gotten very few sable merles. The first breeding I did resulted in 1 tri, 1 tri factored sable, 1 Blue merle and 1 sable merle (he had a blue eye).
The second blue to sable breeding I did resulted in NOT ONE sable merle. There was 1 blue bitch, 1 tri bitch, 1 tri dog, 1 sable dog and 1 sable bitch.... neither were sable merles, as was obvious at birth because they were such dark sables they were almost black (Mahogany).
Then when I bred Paris for the first time, my CH smooth tri-factored sable girl to a blue merle male and got 6 puppies.. none were sable merles... 4 tri factored sables, 2 blues. I might also add that Paris was the sable bitch from the first litter I mentioned above where I had I had 1 sable merle male, 1 sable, 1 tri and 1 blue in the litter. So Paris is from a blue x sable breeding.
Now the litter I had the most sable merles in was actually when I bred Paris to a tri-factored sable merle stud. Of 8 puppies in the litter, 4 were sable merles, 2 were blue merles, 1 was a tri-color and 1 was a regular sable.
The puppy whose head is being laid on by his brother, is the regular sable, probably pure for sable, but he is for sure not a sable merle. If I can find a better picture I will post it, but I lost alot of photos when my computer crashed a couple months ago.
I kept a sable merle bitch from this litter, who is just over 18 months old.. aside from her ears (and her blue eye) her body doesn't look sable merle at all anymore, her ears are still a little spotted though. I THNK She would be the one on the far left in this photo.
Also, IF I had to pick a favorite color, sable merles, or sables in general would certainly not be it, even though most of the collies I currently own are sables.. color is one of the last things I consider, 6 of the 8 dogs I own are sables...however I would LOVE to get another nice tri bitch, but that has yet to happen..lol