This can be caused by a number of factors, especially if they are grown outside. Outside there are many things which can affect the health and appearrance of the pistils. Bugs crawling over them, foliar feeding them with a bit of a hot solution, dust, dirt and such can effect their health also. Unless they totally recede you should be alright, most plants go through stages where some die off and are replaced by new calyx. One other thing it could be but hopefully not and that is the the pistils received pollen and are reacting to that. Don't know enough about your grow to say much of anything else.
Welcome to RIU, I hope you find this site to be to your liking and helpful.
Happy Growing
Cane