My opinion is that it is just excess black with no where else to go. Perhaps as we develop more strains of Reds with ticking in the female neck feathers, the pepper problem will slowly disappear. Let your young started chicks that have excellent type but have slate, smut, or pepper mature before deciding to get rid of them. Let your young started chicks that have slate or smut mature to see if they will absorb the slate or smut in their undercolor. Compensate in your matings, the strength or weakness of one mate to the other mate.
Pullets seem to have more pepper in their wings than cockerels.
Males are more likely to have smut or slate in their undercolor than females. The easy way out results in loss of dark surface color as well as dissipation of beak and leg color. I hope through my research and the findings in my breeding pens, you can better understand this puzzling obstacle in breeding reds.