You can also see that color and markings in the semi-feral mutt dogs in Thailand and Egypt, with no Rhodesian Ridgebacks around.
Crossing various widespread, common breeds brings out a surprising variety, including some surprising colors and markings. I would generally assume the parents are some of the commonly kept breeds in the area, especially if there is a group of breeds that runs loose more and doesn't get neutered.
In one area of my town, that's pit bull dogs. In another area, it's the affluent yuppie dog breeds, mostly Golden Retrievers and Akitas. Not neutering pets and letting them run loose, in other words, 'free puppies!', dumped dogs and ownerless pups wandering around, is common to many different groups of dog owners.
The most populous breeds of dogs in the US, are probably the fighting breeds, then Golden Retrievers, Rottweilers, and Labrador Retrievers. I'm guessing, because I don't know that anyone has really accurate statistics of licensed and unlicensed dogs(not sure how they could track unlicensed dogs).