I think they can be purebred, as well as black and tan labs and other mismarks. As long as they are purebred and fit the standard I still won't understand not accepting them. And you won't understand me. I am into working dogs. Working dogs were once bred purely for type, concern for colour wasn't important. Colour alone doesn't affect the dog.
Actually I wanted to know information about your breeding operation. (put any breeder in front of me and the first thing I want to know is everything about them) Pictures of some dogs tells me nothing about a breeder. You could be the best breeder in the world or the worst for all I know from pictures. I wish you'd have posted some stacked photos of your dogs. They look good but a bit overweight for the type. The last photo looks like there is something wrong with the dog's back legs, but its probably the way it is standing.