That is a totally normal reaction when you begin flushing out nutrients. The plants are "turning" just like leaves in fall season. Let them it is perfectly normal they will even dry up and fall off once they run out of life. You want this. Usually a general rule for the 12 different strains I've grown over the last 2 years has been wait until the pistols (hairs) are orange before you start flushing, usually trichomes become nice and ripe 10-20 days after the pistols have all aged to a nice orange color.