It used to be that less brake dust equated to harder pads equated to less pad wear equated to more rotor wear (friction has to wear something down! if you make the pad harder than the rotor, guess which wears out).
Not sure that holds true anymore - I have a pickup that went 100K without needing pads OR rotors!
it also has to do with brake pad materials. If it's semi-metalic, it tends to be black dusts. The so called "ceramic" brake pads have light colored dusts and are much less visible. They are usually less effective but quiet and suitable for street use