Color isn't an effective indicator. Oil will quickly get dark from the combustion cycle. What matters is the oils retention of its characteristics. Does it retain spec'ed viscosity and are the all important additives still present. Heat does, over time, reduce both viscosity and additives, but neither can be determined by color of the oil.
This is why we have the general "3k" for conventional and 5k or greater for Synthetic. The reality is, even conventional oil is light years ahead of where it was 30 years ago and can usually go for longer; but as it is very hard to determine the shape of oil without a complex chemical analysis (done in a lab)... we stick with these figures.
I talked to a Toyota tech while getting a filter my last oil change; and he told me new Toyota's that run synthetic have a 10k oil change interval. New BMW's are at 15k. Synthetic oil simply does not break down. Even though my maint light comes on at 5k, I have moved to a 7500k change cycle. The toyota tech told me this is what toyota quietly recommends on most modern but older Toyotas. Not talking a 96, talking a vehicle built within the past 10 years.
Anyway. Not really to counter anything said. Since it is impractical to have your oil chemically tested before every oil change; 3k for Conventional is still a pretty good point. That said, even conventional oil in a correctly operating engine, should honestly last 4k or more. Where that line stops, is obviously determined by the quality of the oil.
