I think they come and go heightened emotions they get darker mostly when breeding the back half of body gets darker. they should go away for most part though
A couple of the vendors on here have them from time to time. I have about 350-400 fry from the pair pictured above that are getting shipped to one of the vendors next week. They are only about 3/8 of an inch though, so it will probably be a couple of months before he has them listed for sale.