No, dark color is not necessarily a good predictor of the state of a perfume. A perfume could be clear and be completely off (or, for that matters, diluted or faked). Or some perfume could be darker because concentrated, but the content be reasonably ok, especially in the basenotes. Plus, one would need to know what the original color was.
There is simply no way to know the quality of a vintage perfume without smelling it - a reason why I never bid too much on ebay (and consequently never manage to get Chanels or Guerlains).
Usual indicators one could look at are whether it's in a box (often better because it could mean it hasn't cooked for years in bright light), whether it's sealed (in the case of an extrait - perhaps it means less exposure to air), whether it's an airtight bottle (again, because of exposure to air). And color as well. But these are all imperfectly correlated to the quality.