How big are your pots? Are they in the sun or shade? And can you water daily when it’s hot and dry?
Eucomis (pineapple lilies) bulbs you can put in pots outside now. The flowers last forever and are slug proof.
Crocosmia grow well anywhere. You may need to give them support when they flower, as they tend to flop a bit..
Cannas if you have very large pots (at least 30cm per rhizome). You can start them indoors now, or wait a couple of months if planting outside as they need heat to get started. They will flower late summer, but in the meantime, the foliage is spectacular.
Freesias, plant and put the pots in a cool, dark shed for 3-4 weeks, then bring outside.
You could also plant perennial bare roots now, like echinacea (for sun) or dicentra (for shade).
I think garden centres and bargain shops (eg the Range, Home bargains) have lilies in the green for sale now, if you want colour soon, don’t mind the scent and are not allergic to pollen.