the hairs should be getting red, and the calyx should be swelling up around the hairs... don't worry about the color of the hairs, go by the appearance of the trichomes. they are the crystals you see. get yourself a 30X - 100X jewelers loupe (magnifying glass) and look to see if they are clear, cloudy, or amber. when you reach somewhere between cloudy and amber your done. time to chop.
probably more info than you wanted, but the nugs should start getting a lil fatter and start stacking upwards...