Sulphur is yellow but it turns oil black. It also smells incredibly like an eggy fart when burnt, but that's another matter
I think "sulfur" is the American spelling - is it not? These are the people that park on the driveway and drive on the parkway and are therefore not to be trusted.
Whatever it is that turns the oil black (and opinion is divided in this matter), our troubled poster has nothing to worry about, it's a well known fact that oil in a diesel engine goes black almost immediately, it's not an indicator that the oil was not changed.