I thought it was when there is a base colour (ie red) with other colours (blacks, browns, greys, fawns etc) mixed into it but not patchy like brindle, more blended in? I'm just guessing though!
I'd always understood smut to be a base colour with black overlay, not stripes like brindle and not clear black tipping but just a 'muddying' of the base colour :-)
Smut or dusting (in Bullmastiffs) is when the base coat fawn or red has black fur laced through its coat usually on the face and chest, it happens regually in this breed but is not desirable as the colour is to be pure. The dogs look as if they have rolled in a coal pit.
Thank you for that carolyn. Although it now poses a bigger question as to what colour this pup actually is!! She is red fawn with a black "dusting" over her paws, ears, shoulders and tail. Suppose I will have to go for red fawn and hope for the best!