Spring color is important to any border
Flowering almond bush, jonquils, tulips and phlox
I love roses, so I tend to build my borders around them. I also believe that a mixed border isn't complete unless the roses are under planted with compatible plants.
Felicite Parmentier rose under planted with lily of the valley
Pink rose as a perfect complement to purple violets
Vertical interest gives the border added dimensions. I have two climbing roses on bamboo teepees in my garden
On one I have a New Dawn rose interplanted with a Nelly Moser clematis. They have a bloom overlap period of about 2 weeks and are gorgeous then!
Many other plants can add vertical interest
Echinops, hollyhocks, stock, and lupines
Soften the edges of your garden by letting plants spill over onto the sidewalk. Just make sure that the plants smell good so that when people brush up against them they smell something divine.
Catmint, pinks, and lavender are all great choices.
Fill in the garden with plants that fit into your border's color palate
Veronica, pink Asiatic lilies, salvia, Gertrude Jekyll rose, purple scabiosa, cosmos, perennial geranium, peonies, and Russian sage
And finally, add a few doses of unexpected color
Hyperion day lily, red geum, red scabiosa, and yellow geum.
So these pictures are based on the colors that I like to see in my garden. If you like yellow and purple, then by all means used yellow and purple flowers. I also use flowers that I can abuse a little, that tolerate pretty poor soil (you could mold a mug out of mine, even with all of the amending) and that can take slightly dry conditions. Easy care is my motto!
So get out there and plant! Create the border of your dreams! Live the gardener's life - it's good for you!