Terminals on Electrical Wiring

I have a question about the tab type terminals commonly found on the wiring inside appliances and electronics. I notice that about half of these connectors I come across have the golden brass color and the other half have the grayish aluminum color. Do these terminals vary in the material they are made of or are they typically all brass? Can I just put these all together in one bucket or should I be separating/scratch testing the gray ones?
Any help you can offer is greatly appreciated!