That's an indicator they're getting ready to start laying. In the winter time or before molt, you may see their combs going back to pink, HOWEVER, a pink comb can also be a sign of anemia, so pay attention to how the bird acts, too. They may start showing some interest in your nest boxes, trying one out, then another. Another sign, if your birds are hand-tamed, is when you reach to touch or pet her, she squats. That's a sign of submission indicating that she'd accept a rooster.