for a color to be natural, it would have to appear at least in the 25% of the general population.

A colour can't appear that often when only 3 colours are line bred by show breeders. If you put all colours of labs together and let them breed without human interference for enough time to overcome many years of line breeding you'd come across a pertenent palette of natural colouring. Of course this is never going to happen, nor should it. I'm not saying that these colors will be 25% of the population, I'm just saying they would come along and still be purebred and still be labradors.