They aren't 'supposed' to be white; pure white teeth are bleached teeth, it's not natural.
(I suppose truly natural is also unhealthy, but my point is perfectly healthy teeth can be - and are, without other non-health-improving intervention - less than pure white.)
(I suppose truly natural is also unhealthy, but my point is perfectly healthy teeth can be - and are, without other non-health-improving intervention - less than pure white.)