For constant color, I suggest you consider planting a mix of annuals and perennials. The perennials can be the backbone of the garden, and you can fill in around them (especially the first couple of years as the perennials get established) with annuals. Many perennials do have a long season of bloom or repeat cycles of bloom. Some to consider for your sunny garden are red valerian (Centranthus ruber), Jolly Bee geranium (Geranium 'Jolly Bee'), reblooming daylilies (Hemerocallis 'Miss Mary Mary', 'Stella de' Oro', and 'Happy Returns'), false sunflower (Heliopsis helianthoides), Coronation Gold yarrow (Achillea 'Coronation Gold'), Stokes' aster (Stokesia laevis), Tennessee coneflower (Echinacea tennesseensis), black-eyed Susan (Rudbeckia hirta), sedum (Sedum 'Matrona' or 'Brilliant'), Russian sage (Perovskia atriplicifolia), and catmint (Nepeta Yfaassenii). I'd also mix in a few crocus, daffodil, and tulip bulbs for spring interest. After the bulbs fade, the perennials will take over and provide color. Also, include foliage plants that look good all year long without flowering. Artemisia absinthium 'Lambrook Silver' is one.