The red color is default provided by Pega in the class ".notification-count".
You can explore more in the developer tools. Attaching a sample DOM snippet (notification.png).
You can either override this class in userworkform (less preferred from Pega 8) or you can edit the available rule as shown in the attachment (OOTB CSS). Change line number 6 and 20 and make it black (written below) or any other color code: