So you want these to be outdoors all year? I suspect Japanese maples were born for this application, but the roots will need protection in the winter. I think that will be true of virtually any container tree in your zone. Certain varieties are well adapted to colder zones.

Some magnolias might work, as well as some varieties of crab apple. A lot of conifers will work, but they're kind of boring. No matter what you choose, I'd still provide it with root protection in the winter. Look for trees a couple zones hardier than zone 5, if you want to be cautious.

Yes, I want them outdoors year round. I do have a garage, but it still is freezing in there, too. No room in the house.

Since I just got rid of the arborvitaes, I definitely want color! I'm open to how the color can appear, though.

My main concern is things that won't get too large - or does the nature of being in a pot help keep them smaller?

It also doesn't have to be a tree...could be something tree-like, I suppose. Hardy hibiscus maybe? Lilacs?

I have Korean lilac 'Miss Kim'(shrub) growing in container for past 5 yrs. It is only abt. 4'tall, I believe because it's in the container. It may be the dwarf version thou, that grows to only abt. 5'. Otherwise they grow to abt. 8' tall. I love it's fragrance at blooming time, and little maintenance. You can get it as a standard too.
Fringe tree may be interesting (to z.4 I think).
Pagoda dogwood? (to z.4) - not sure how wide these 2 would grow.
How about corkskrew hazel (also to z.4).
You can also grow fruit trees...

Rina, I like the idea of fruit trees.

When you grow trees in containers, how to you provide root protection?

The Korean lilac is in a wooden box build by me. I only had some 1/2" plywood that I used. Did not know anything about growing trees in containers. But I thought that I should insulate it. So I put 1/2" styrofoam on inside assuming it would be good insulation.
I really don't think so now, the styrofoam was just thin sheets. I do not think it helped any. Maybe if it was the proper used to insulate houses then it would helped.

I have lots of others in containers that are just large plastic pots (recycled, I either pick them up when somebody does landscaping or you can get them from garden centres). Usually I don't do anything.
I have potted a tree peony last year; in beggining of winter I inserted it into larger pot & 'stuffed' with shreded leaves between the pots. The tree peony doubled in size. I do not know if that 'insulation' was necessary or not, but the plant was bit pricey & I didn't want to loose it.

I would think that wooden containers may provide some insulation? - better than plastic or metal.

And I absolutely believe that good drainage is crucial - so the roots don't sit in water.

I would think that wrapping container in burlap may help too. Just like surrounding container with shredded leaves-just need to make sure they don't blow away.
I am planning on getting some fruit trees, maybe some columnar (they are more $$$ but take much less space), or dwarfs. I believe they are hardy to z4.

I know that some burry containers in ground for the winter. I don't know if that is better protection or not.

I am not an expert in growing, but am not afraid to try anything.
And as g87 mentioned, just make sure tree is minimum 1 zone hardier than your zone.

I hope someone with more & good experience will chime in...Rina

Crabapples or apples would be the best choice, IMHO, for the OP based on all of the information presented in this thread. Malus would do fine permanently in large containers with a bit of soil replacement every 3-4 years and frequent fertilization (Osmocote would do it). The advantage would be that these are hardy enough not to have to do anything with them in the winter -- just be sure the containers you select are also freeze-proof so they don't break up with freeze and thaw cycles.