Thank you very much for choosing our theme and contacting us.
As soon as you add a category to the menu, save it and you will see all the custom fields. Also, check your screen options in the Appearance > Menu, make sure all the options are checked http://prntscr.com/k1i6im
Enter the category element and change the color: http://prntscr.com/k1nsnt
Take into consideration that there is also the mobile view of your header http://prntscr.com/iyd3v3 check all the elements there.
.
