Be carful to choose perennials that are in scale with your area. Look for contrasting shapes to put near one another. Flower color is only one of the elements to consider. Decide if you want a monochromatic garden with shades of the same color, a color coordinated garden, or a wild mixed garden. Look to place pleasing and contrasting foliage colors agaist one another.
Texture is another important element in a pleasing arrangement.
Look for at least a few plants with strongly contrasting texture. Put them together here at the nursery and if you like them, try them. If the plant arrangemet doesn't work in your yard most perennials are easily moved. A few are evergreen and show all winter long. One with tiny almost white flowers, and one with 2 inch lavender flowers on clumping plant.