rumor has it that 1 micron is roughly the size of diesel soot.However you could filter the oil until you were blue in the face and it would still come out black.Oil turns black because of the high temps of an engine.An oil could be very clean but still be black because it was overheated.so the blackness of an oil is an indicator of how long it was in an engine.However, ever notice how the oil from a vehicle seems way thicker than when its filtered/centrifuged? Its thicker because of all the carbon particles in suspension.filtering/centrifuging removes most of that carbon in suspension.
so rather than color,a better indication of carbon content is viscosity.clean oil-flows like new oil.Dirty oil flows like tar.