Any combination to include a blooming plant or just one plant in a colorful pot would be a burst of color. Put another upturned pot or stacked bricks to bring pot to level desired.

I mostly use groundcovers as spillers.
Use a bulb planter to make your plugs if desired. (make hole in planter soil, go make a plug of groundcover to carry back to planter to plop in the hole. Then go back to garden to fill the empty spot with compost and it will fill back in quickly this time of year. **Local independent nurseries have a wide selection of groundcover plants that will usually work great in easy care containers.
vinca minor esp. variegated, but all work (tuck in some of the trailing stems at least 1x a year to root & fill out more)
golden creeping jenny
various ajuga (if planted prior to bloom & late spring spreading it just flows out of the pot)
variegated bishops weed, Aegopodium podagraria *as a solo plant in pot or with a bright blooming annual looks great if you can keep it moist all summer. Cut back after a hot spell & it fills out again. Over time roots crowd the pot, so replanting will renew it again. Cover with evergreen boughs in late fall for winter interest. Sometimes I even insert the branches and it doesn't seem to bother the groundcover any. I cut blooms off because I don't want it to reseed in the garden.

fillers:
lamium, spotted dead nettle esp. white foliage marked ones
sweet woodruff
bergenia
hosta, esp big blue leaved ones for that round full pot look!
Sarcococca, sweet box
Serbian bellflower
pachysandra, variegated or solid green(sweet fragrant blooms early spring)
columbine, any (cut back hard after bloom & foliage looks good, some have darker stems, too)
lady's mantle (if you don't like the airy yellow blooms cut the stalk off & enjoy the large leaves)
masterwort, Astrantia -- long blooming June-frost here, some are shorter & ideal for pots
annuals like impatiens, nicotania, begonias, or coleus (some can be overwintered inside)
upright fuchsia with interesting foliage like Aurea, Golden Herald or Autumnale
many hardy geraniums do fine in pots as long as you keep them moist (blooming is less in shade, but some go all summer long or rebloom if cut back hard after flowering

accent grass
Acorus, sweet flag esp the golden one (slow grower)
Carex, Ice Dance or orange
Hakonechloa, Japanese forest grass esp Aureola
ribbon grass, Phalaris in a sunken gallon pot so you can remove it midsummer when it looks shabby (mine always does no matter what, but I still love it in early spring and summer)
fern - especially if you have any native to your area (I've used our lady ferns for years in L. pots)

Hope that helps,
Corrine