But don't you see, that's not the point. GSD's can come in solid black (if I recall, solid black isn't showable, but still, that's what some people think they want). In fact, I daresay that as unusual as solid black GSD's are, there are waaaaay more of them than there are fawns, simply because some people
select for solid black, while the tendency is to breed
away from fawn. But you might as well say, "You have a solid black dog. GSD's can come in solid black, therefore your dog must be at least part GSD. Never mind that everything else about the dog shouts "Newfoundland" (including his pedigree), he is a color that occasionally shows up on GSD's, therefore he must have some GSD in his background."
We were looking at a dog that had a slight resemblance to one breed, and a pretty marked resemblance to another breed. It is
possible that she is a "Heinz 57" with a little bit of GSD somewhere in the mix, but it is also possible that she has no Shepherd in her at all, but a lot of the breed she more nearly resembles, the Malinois. IMO, the resemblance is strong enough that she might even be a purebred (if poorly bred) Malinois.
Granted, the Malinois probably wouldn't turn up in a
Family Feud survey, but people who use dogs in security work are usually well acquainted with the breed. In fact, given the lighter frame and shorter coat of the Malinois, I'd think they might be better suited to working in a hot climate like Mexico than a GSD would be, and therefore more likely to be found there.
If you want to make a game of trying to guess at dog breeds in canines of unknown ancestry, hey, have fun with that. But if you want to be "good at it" (as you claim to be), maybe you should expand your knowledge base a bit more.