i had my white classic chanel lambskin for 15 yrs and had always kept it in the black dustbag that was provided, had no issues till today and my white bag is still as white as the 1st day i brought it home. Actually the black dustbag helps to prevent the light color leather esp. white from turning yellow. I dun understand why but it was my SA who told me. But i guess she was not lying as like i says, my bag is still white till today, not even a speck of discoloration anywhere and i only maintain the leather by nourishing it with mink oil maybe once every 6 mths. I do that for my white shoes and other light color leather bags too.
Hope that helps.