An interesting thing about colour is if you look at old artworks of mastiffs they're most often white with patches. Including those of Lyme hall from which the English mastiff breed originated. Only occasionally are they fawn (and also occasionally brindle or black, but most commonly white with patches). Old photographs of boerboels and presa canarios and Alanos and fila brasileiros also often show white with patches dogs. 

There was a trend in the 1700s and 1800s with the ancestors of the bull mastiff, the gameskeepers night dogs, to breed them black or brindle and avoid white so they couldn't easily be seen by the poachers they were tasked with hunting, but even this would indicate their stock foundations were often white. That they had to breed against it. 

Where the dominance of fawn and black masked came from I'm not sure but I do know it seems very recent and I suspect it's connected to narrow gene pools used in the creation of the pure kennel club breeds of bullmastiff and English mastiff. The same few individual dogs, who happened to be fawn, overused in the establishing of these breeds in the late 1800s and early 1900s, and these dogs then played a big role in creating the tosa and boerboel. A similar phenomenon, but with blue and black, happened in the establishment of neos and cane corsos as breeds, and then red with dogue de bordeauxs. But before kennel clubs I think all of the above were a mixture of colours, most often white with patches (of brindle, black, red or tan) and sometimes black, brindle, red or fawn. This is what we see with old paintings (and some old photos) depicting the working ancestors of these breeds regardless of country.