Do you mean between black and light grey? Light grey pins have always been the kind you use for rotating connections (low friction), whereas black was for non-rotating ones (high friction). Newer blue pins are also high friction, IIRC. I haven't bought new lego technic in a while, so I don't know if there's been any new colours added

EDIT: I think I also had some dark grey pins, but I don't remember if they were high or low friction