Because of your location in Montana, it is impossible for the traditional warm season grasses to survive. What you likely have is Kentucky bluegrass which goes dormant and turns gray in the winter. It can go dormant in the summer, too, if it gets real hot and you don't water. The grass that remains green all winter is a tall fescue type of grass. The solution to a more even looking lawn is not that difficult, but I would like to know the answers to a couple questions. Your normal care routine might suggest a more perfect solution for you.

What is your summer watering schedule (how long and how often?)
How high/low do you mow and how often?
What do you fertilize with and how often?

I'm not sure it matters but where in MT do you live? Can you describe your soil (sandy, rocky, clayey, gravelly, loamy, etc.)