Artists know that a light color only looks that way when surrounded by dark colors to give it contrast. Likewise, artists use form, line and texture to keep things interesting and depict their subject.
These things work in the landscape, too. Most of the time, we start with foliage color, since it’s present most or all of the year. Flowers are a bonus, unless the plant happens to be a variety with great flowers and only so-so vegetation.
When working with color, there are several approaches. Complimentary contrast generates a lot of visual energy, but can get a bit jarring especially if you’re looking for a peaceful landscape. This contrast uses colors that are opposite each other on the color wheel (the one we learned about in grammar school): red and green, orange and blue, yellow and violet… Note that red flowers backed by green leaves automatically have a complimentary contrast.
Another approach is to use analogous colors – where they share a common primary color, for example. A series of blues, greens and blue-grays will be soothing. A series of yellows, oranges and reds will splash color across an area of the landscape. If you have the space, you can even create beds where these two schemes touch to provide more drama.
Another artist trick is to place a color against a gray background. This makes the color appear brighter and more intense. If there is a value contrast – dark against light – then the effect will be even more profound.
Texture contrasts can be fun, too. Pairing a bold, architectural plant with a lacy one will enhance the structure of the architectural plant. There can also be a color contrast in play here, too.
Plant forms should work together, too. Some are erect, others loose. Some plants look more like arcs, others lines, others more or less amorphous. You can set up a progression of arcs, then break it up with something linear and vertical. Or you can use only arcs and create interest through foliage and flower color.
Remember too that your hardscape can work along with all this, so if you want loose forms and informality in the planting design this can be balanced by bold forms in the hardscape.