Is the garden in sun or shade? Is the soil dry or moist?
Some ideas:
For moist positions in a fairly sunny position, candelabra primulas would give lots of colour; if it's shadier you could have astrantias (not bright but cool and elegant).
For drier spots in sun, try pinks or helianthemums

You could certainly get a good show next year from clematis on an archway if they were planted asap. You want early large-flowered varieties such as Warsaw Nike (rich purple), Comtesse de Bouchard (pink), Niobe (red), General Sikorski (blue) or Marie Boisselot (white).
Roses are reliable but probably won't have made enough growth to cover the arch by next year.