I'm sorry if I'm not capable of answering the question, but let me share my though.
the color of the metal changed for several reasons, including tarnish. because it is exposed to air, so its color had changed. or the second is because the coating/plating. Because (maybe its been touch by many of customer), the plating begins to wear off and reveal the original color.
But seen from his many bracelets that change, the possibility for exposure to the outside air, while partly of the bracelet stored or wrapped in plastic or its packaging. So the color change only a part of the bracelet (that exposed to air)
Correct me if I'm wrong