It's also a myth that all raw fed dogs that have bones have perfect teeth - I know of many that have dentals at the vets as they get older.
Jarvis is nearly 10, he has had a dental although the vet said he didn't really need it - he was going under a GA as he had knocked a front top incisor and it was going rotten so had to be be extracted so we did the whole lot while he was under. Apart from a bit of tartar on his canines they were all good. He is fed part raw, part kibble.
His half sister owned by a family member has perfectly white teeth and she is 9 - and was fed Bakers until she was 6.
As others have said Its more luck with some that tooth management
With the tartar on Jarv's canines I have used petzlife gel and it really does work. After a couple of weeks the buildup just came away.
I would say that young dogs should have good white teeth until they're about 6 - the age starts taking their toll. The brown is very likely staining from the food and I don't think its detrimental to tooth health - much like tea/coffee stains on human teeth.