I looked up the Bullmastiff standard and it looks like it’s been this way since 1992:
Color: Red, fawn, or brindle. Except for a very small white spot on the chest, white marking is
considered a fault.
English Mastiff since 1991:
Color: Fawn, apricot, or brindle. Brindle should have fawn or apricot as a background color
which should be completely covered with very dark stripes. Muzzle, ears, and nose must be dark
in color, the blacker the better, with similar color tone around the eye orbits and extending
upward between them. A small patch of white on the chest is permitted. Faults - Excessive white
on the chest or white on any other part of the body. Mask, ears, or nose lacking dark pigment.
Cane Corso since 2009:
Color: Acceptable colors are black, lighter and darker shades of gray, lighter and darker shades
of fawn, and red. Brindling is allowed on all of these colors. Solid fawn and red, including
lighter and darker shades, have a black or gray mask. The mask does not go beyond the eyes.
There may be a white patch on the chest, throat, chin, backs of the pasterns, and on the toes.
Disqualification - Any color with tan pattern markings as seen in black-and-tan breeds.
There are also preferences or requirements for eye color, lip color, nail color etc. So a white nail on a dark toe is a nope for the show ring. Or at least would make it harder to finish a dog.