Pistils can turns brown for many reasons, check you plants to see if there is an hermie in the group ( can be a plant with 1 or 2 pollen sacs ) . The fact that the rain is falling on them can make them get brown faster, and generally it happens when you spread something on the buds . The last reason could be age of the pistols , maybe they are old and browning because your plant are producing new white pistils so she don't need the oldest ones