Basic cube here. Add material. Go to edit buttons. There’s a box there that says O mat O. Hit the New button, now it says 1 Mat 1. Now on your cube, select the verts on one side so that face is selected. 1 Mat 1 is your base color, so now hit New and then hit Assign. Now the whole cube is under 2 Mat 1, except for that one face that’s under 2 Mat 2. Add new again and select a different side of the cube and hit assign again to add more materials.
Now go to Materials. Notice the little box that says 2 Mat 2. You can change between 2 Mat 1 and 2 Mat 2 etc there. Look above and to the left and look at the MA:Material box. There’s a 2 next to it because both 2 Mat 1 and 2 Mat 2 are using it. Click the 2 there and select yes. Now you can flip between 2 Mat 1 and 2 mat 2 and change the materials for each.