Although some black nosed doodles lighten with time or in the winter, true pink, rose, or liver nosed dog are similar genetically (as far as color genetics) and are such from birth and on. There is no change in the nose pigment at any time. If you look at a true pink, rose, liver nosed dog, you'll see that their lips and eyeliner matches the color of their nose and is equally light colored.
Same goes for chocolate dogs. The only difference is that while cream, red, apricot, and white can have both kinds of noses...chocolate dogs always have brown noses (you'll never see a chocolate doodle with a black nose).
To read more about the genetic combinations that make up coat color and nose/lips color see these links:
As Karen and I were discussing in the comments section of this group, neither the lab or the poodle are 'supposed to' have the light colored noses. According to the breed standard it is a fault. In labs, what I've read is that hunters found the light noses (and corresponding amber eyes) to be too spooky. Silly hunters!
But honestly I NEVER liked the light noses on light colored dogs UNTIL I got Rosco. Now they are my favorite on cream doodles =)