Pearls will take on a dull yellowish brown appearance if they are allowed to contact perfumes, hair sprays and excessive perspiration etc. There is a saying, that pearls are the last thing on, on your way out, and the first thing off, when you arrive home.
As pearls may dry out and crack with time they should be cleaned with a moist soft cloth from time to time. When caring for your pearls it is best not to wet the string or cord, as it may rot over time. The pearl itself is better off in water than air!
If you wear your pearl necklace or bracelet frequently then restringing is advisable every year or two, both from the security viewpoint, and a dirty brown string does nothing to enhance the beauty of your pearls.