Considering I'm a smoker and coffee drinker, me teeth are a decent color. all I know is that last month I switched from coffee to tea and now ALL my coffee mugs have wicked stains. Haven't noticed a change in my teeth, but it could be subtle. I use a toothpaste with whitener in it.
Anyway, my kids' teeth get pretty yellow despite regular brushing and avoiding foods with dyes. Whenever I buy, and they start chewing Trident after every meal, their teeth whiten up. HOWEVER, this only happens with the Trident made with XYLITOL. Sometimes it's marked on the front of the package, other times you have to pull out the microscope and read the ingredients.
XYLITOL is awesome stuff. It's an artificial sweetener that is actually GOOD for your teeth - it actually does a bit of 'repair work' to enamel.