change in color of dogs nose

Question
what causes a dogs nose (we have a yellow lab) to change in color.  Our dogs nose was black as a puppy but has turned to a pinkish brown from 7 or so months on.  She is now almost 1 yr old.

Answer
It is a lack of sunlight.  My Aster was born in October.  Her nose stayed black most of her first year, but was fading the next fall when we left her at dog guide school.  It was unusual, but after her training she was given to a lady in a nearby town.  The sun gives the lady migraines, so she doesn't go out much in the sunlight.  Aster's noise stayed pink all the time she worked as a dog guide.  She is retired now and lives with us again.  I thought maybe with the change of lifestyle, it might turn black last summer, but it didn't.  Her mother's nose used to be black in the summer, but as her activity has declined with age, it is now pink all year around.  I just checked a picture from Aster's retirement party in October 02.  Of the 6 yellow Labs in it, 5 have pink or partly pink noses.  Just one seemed to have a solid black nose.  

Some yellow Labs have black noses all year around.  Some are pink year around.  I think most of them vary with the sunlight.  It is fairly common for puppies to be born with black noses, and stay black their first winter.