With out registration you cant, I mean AKC not universal kennel crap, or continental kennel crap, a real registry, if you think they may be the reuslt of a cross then you can get the AKC involved I think.
Did you get your chi from a "knowledgeable" breeder...I'll use that term rather than "reputable" as your definition of a reputable breeder could be very different from mine. I also know some very knowledgeable breeders that I could never consider reputable. There are soooo many permissible colors in chis, and I am not familiar with all of them as I am with minpins (and double-coated?? not at all). However, the genetic principles of the "blue" color designation are the same in both breeds. A true blue can vary from a fairly light steely grey to a very rich, dark (almost black) charcoal grey color; they will also have bluish grey eye rims, nose and foot pads. I always associate sable with a brownish cast. A sable chi I think should have a black nose and footpads. A chocolate would have brown (could vary from light to dark) or even pinkish brown nose and footpads. But the "blue" is definitely grey.
If your breeder knows her colors, she should have listed it properly on her registration papers. And for Ruffian, please remember that just because a dog is AKC registered doesn't mean the color was listed properly...they are a registry ONLY. It is up to the breeder to list the proper color. Now by saying that, I believe AKC is the premier registry, and the only other American registry other than breed specific registries I consider reputable is the United Kennel Club. In Canada the only reputable registry I am aware of is the Canadian Kennel Club.
Hope this helps you determine if her color is listed properly.
When she put true blue chihuahua, I thought she was wondering if it was a cross or not, which is why I brought up the registry, because that way if there was any question she would have some recourse. It was not until a later post, after mine at that, that she brought up the shade of blue her dog is.