Our Lowes had a fair selection available. They had TONS of vinca, a great mid-summer potted plant. They also had plenty of begonias with a variety of colorful leaves.

GGG

coleus - they can handle sun or shade if you need to "coordinate" the containers. i planted a flat of the chartreuse colored ones and those that are in full sun turned as yellow as any marigold flower i have seen.
i always thought of them as a plant that did not perform well in full sun but they plant them at the shopping centers and mall here in auburn in all-day sun and they are really big and colorful (obviously, they have an irrigation system for the beds).
it's probably late enough in the season that you won't have to bother with pinching the blooms out.

For the planter with full sun:

Madagascar Periwinkle (as suggest above--come in lavender, pink and white)
Gerbera daisy
Gazania (African daisy)
Portulaca
Ice plant (flowering succulent)
Dusty miller (as contrast plant)
Marigolds
Plumbago (pale blue flowers)
Tropical Hibiscus (depending on size of planter)
Orleander (again, depending on size of the pot--but can bloom relatively small)
Bougainvillea
Mandevilla vine
Dipladenia(similar to Mandevilla vine but easier to overwinter indoors)
Miniature roses?? (great in containers but not lovers of continuously high, midsummer heat)

For the pot that's shaded:

Caladium (grown for colorful foliage--not flowers)
New Guinea impatients
rex begonia (foliage)
alocasia (colorful foliage)

Sorry, I am beginning to draw a blank for heat and shade tolerant flowering plants. As you notice, must of the ones for the shaded planter are actual foliage (not grown primarily for their flowers). Most of the ones I can come up
with are really tropicals and will need attention to watering (Spathyphyllum for example).

The real problem is what you can still find salvageable in HD or Lowe's at this time of year. Most of the annuals are
gone. I do recall seeing a sea of caladiums on a recent visit to HD. (Yes, HD's are the same all over the world!)