My white s/s 06 (tag w/ Y), it got SO DIRTY from traveling and just out and about, in the streets in ny & france, the handles were turning this nasty gray and the bag started looking old yellowish white from all the city pollution/dirt, etc.
Neimans told me to take it to Shoe Lab and the Neimans in Vegas uses Shoe Lab. So I did and they ended up having to re-dye the entire bag white...at first, i wasn't too keen on re-dying my bag, but i'm happy with the result, looks new and only took one to two weeks to "break it in." The leather is back to really soft and you can't even tell. If you have a white bag and it's really dirty, i say go for it and use Shoe Lab. I only think you can get away with dying the white B bags back to white. I wouldn't do any other colors.