Discoloration due to heredity and dental history.
People are genetically programmed to have lighter or darker teeth, and are predisposed to respond more quickly and severely to staining agents.
Dental caries is a primary cause of unattractive pigmentation(opaque white halo or gray discoloration.
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Fluorosis staining.
It is caused due to excessive intake of fluoride during the development of enamel formation and calcification.
Seen in population where drinking water contains more than 1ppm fluoride concentration. Fluorosis may be:
a) Simple fluorosis staining. Appears as brown pigmentation on a smooth enamel surface.
b) Opaque fluorosis. Appears as flat gray or white flecks on enamel surface.
c) Fluoride staining with pitting. Dark pigmentation with surface defects.
These cases respond to vital tooth bleaching. If stains are set deep into the tooth and are very opaque, then bleaching should be followed by veneers.
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Tetracycline staining.
a) First degree tetracycline staining. Light yellow, brown or gray uniformly distributed throughout the crown, with no evident banding.
b) Second degree staining. Darker or gray uniform staining, with no banding.
c) Third degree staining. Dark gray or blue staining with marked banding.
d) Fourth degree staining. These stains are too dark. You may have to go for veneers to treat such cases

I saw this described in a book (I think it might have been about enzymes) at a herbal place here in town. The pictures in the book really caught my attention, and what you described reminded me of this. I will look for the book next time I go to the herb shop so that I can give you the name of the book and the information. It will probably be sometime next week.