I have seen this before in some of the bigger breeds. I've had Plymouth Rocks and Jersey Giants whose combs did this, and my Delaware rooster (he is a big 9.95 lb boy) has a comb that still turns part purple sometimes, for reasons unknown, eventually fading back to normal. I would bet on a circulatory issue, but that may be applying mammalian logic to a bird system for all I know. With Hurley, I just keep an eye on him when I see his comb's gone purple, make sure he's still moving around and acting normally. I think that's the bigger health indicator most of the time. So, in short, I'm not sure why your hen has a purplish comb, but I've had many with them, and they've all lived normal lives. Hurley himself will be 5 or 6 this year, depending on who you ask.