Slate colored juncos (dark-eyed juncos) are some of the best birds to try out your identifications skills with. The first two photos are ones I took in Duluth; they are of a junco who has not molted from the pre-basic molt. They look more like the "sparrows" they are! Normally by the time they have made it to the metro area in fall migration of their hatch year, there are few individuals who have this plumage.
The next two photos were taken in my yard, so birds were obviously not in hand, but appears to be that they are juvenile females.
Ahh - here is an adult male that looks like what might be pictured with the species.