The "slate-colored" is actually just a subspecies of the Dark-eyed Junco. So you actually have a Dark-eyed Junco (Junco hyemalis) no matter which subspecies it is. The subspecies exist as populations in certain regions of the country. Those listed in my field guide are:

1.) Pink-sided
2.) Oregon
3.) Gray-headed
4.) Slate-colored

Each subspecies looks a little different.

As for the slate-colored (the subspecies I see in my location), the first-winter males look very similar to the adult females (brown instead of slate gray). They can be difficult to age and sex unless it's the adult male, and the adult male is dark slate gray with a white belly and white outer tail feathers. Very distinct plumage on the slate-colored Dark-eyed Junco.