Why does whole wheat dough turn gray after awhile?

Anyone else notice whole wheat soaker or biga dough turning gray where it has contact with air?  I recently made a dough that had a soaker (contained salt, sat out 24 hours on the counter in a cool kitchen) and a biga (in fridge for 24 hours), and they were covered with plastic wrap.  The plastic wrap was not in direct contact with the dough.  Where the dough was exposed to air, it turned gray, but the rest of the dough was normal.  I've seen this kind of thing before.  Though disconcerting, I just continued with the recipe for the final dough, and it seemed fine, the color was very nice.  (The loaf, Mebakes barley bread, is fantastic!)

Why does whole wheat dough turn gray where it contacts the air for a long time?  I have also noticed this with leftover whole wheat dumplings...after a couple of days in the fridge, unless they are immersed in gravy, they sometimes take on a unappetizing grayish cast.  Hmmmm.

Mary Clare in MO