I don't do any pots, because I neglect to water. But, I'm newly addicted to coleus which come in hundreds of different colors and patterns that could highlight almost anything you could come up with. They are also the easiest thing in the world to propagate by cuttings. I'd take a look at them for inspiration.

Martha

Many years, I choose a new color for all of the pots on my deck and spray paint all of the pots to provide a unifying theme. Other years, I've used a repeating annual like red impatiens in about half the pots.

I went so far into this direction, that I don't even have flowers in pots anymore, just in the ground. Pot space is too valuable, to me, to not have plants that look great every day, hence beautiful leaves are required, no waiting for blooms. I made a article about it.

For my something tall plant I use a small piece of canna,I like the shorter kind .In other pots I use red fountain grass,a piece of my Morning Light grass or maybe a Colocasia Illustris black elephant ear.These are among my go to plants to set off my flowers & coleus.
I can hardly wait!
Rose AKA Doris

I'm addicted to coleus, too, and they do great in pots. Look at your local nurseries when it warms up, or you can order some from a big nursery that specializes in them. I don't know if they're shipping them right now, but you could look at the pics and dream.

Here is a link that might be useful: more coleus than you can imagine

Yes coleus does great in pots. I do grow it on the north side of my house. However these are pots on the south side of the house in full direct sun all day. I have tried coleus in them and they got fried from the sun. I was looking for things I could grow in pots in full sun.

Not all Coleus are created equal, and some can handle full sun, but it's hard to know up front without experimenting, or ordering expensive plants or seeds in the mail. They're usually all lumped together in the shade at a store. In zone 3, you probably don't have the time for that, so I don't blame you for not wanting to spend your short summer looking at experiments that may or may not go well.

By name, if you can find Coleus from the "Florida Sun" collection, you won't have to experiment. Florida sun 'Jade' and Florida sun 'Rose.'

These can handle full sun:
Tradescantia zebrina
Tradescantia pallida
Gynura (purple passion)
sweet potato vine might be OK that far north in full sun
If you like bronze color leaves, Perilla Magilla
variegated Vinca vine

(Although none of them are green!)