The dyes used to color pearls can fade over time. Because pearls are porous, they often absorb human oils, makeup and perfume, which can stain and alter the color of the pearls and the dyes used to color them.
Pearl ''treaters'' take advantage of this porosity to achieve finer, more uniform colors with natural extracts and inorganic and/or chemical dyes and are always striving for long-lasting color. Matched strands are easier to create when dyed, and they're more affordable. It is best to buy as many strands of pearls you will need long term, since dye lots change and it will be challenging to find the exact match in a new batch of pearls.