Let's Talk Complements
In visual art and design, complements are those pairs of colors that lie opposite one another on the color wheel. That's the simple definition. Wikipedia and many other sources will tell you that in consideration of the three primary colors (red/yellow/blue, from which all other colors are mixed), complements would be a single color [let's say red] and the mixture of the two remaining primaries [blue+yellow=green]. Draw a line bisecting the wheel on the left--it doesn't matter where you make the bisection, but it must create halves--and the colors you slice will be complements.
What you really need to know is that complementary colors will put the "pop" into a landscape design right quick-like.
The simplest translation of this would be to purchase some plants that have flowers which happen to be complements and plant them right next to each other. Instant Zing! This first photo is something of that variety. We have "Diablo" ninebark, which happens to have a dark purpley leaf, providing the backdrop for a luscious variety of rudbeckia, primarily gold/yellow. Now that exact purpley color is not on the color wheel provided, but you get the idea. Shazaam! It works even better with a plainer rudbeckia that has more blooms on it. Multiple plants. In a huge mass in front of the ninebark. Simply delicious.
Not everything in your garden, however, is a plant, but the colors still matter. Consider the raw stages of a bed reworking, here. A bluish-hued stone path is about to be surrounded by plants with orange blooms. The first of these, some annual zinnias, are in place. Behind the path, adding another layer of height, will be rusty-orange re-blooming daylilies. I can hear my neighbor now-- "THAT's not really blue!" -- and she would be right. These stones are essentially gray. But nearly all grays lean a certain direction--these lean towards blue, making them an ideal complement (not compliment) for orange flowering plants.
Let's consider your evergreens. I know you've got some somewhere. Evergreens are generally... green. So let's cut that wheel in half and what does that lead us to? Red! Lots of different ways to mix your reds and greens. Some reds are so deep and cool that they lean towards blue, and others are much warmer and lean towards orange. Likewise, some greens lean towards blue, while others lean towards yellow. In this example, we have a slightly blue-green juniper (Blue Pacific--doesn't mind aggressive pruning) with a sun coleus that gives us both a yellow-orange and an orangey-red. Next to that is Cherry Brandy rudbeckia, which is a cooler red. In front of both of those is a sedum that stays low--with a blue tinted leaf. Pops off the orange in the coleus nicely!
Finally, let's get away from the plants that are in close proximity and see how complements can be used within a view. In this image we have a cultivar of agastache with yellow-orange blooms that is seen framed against vitex, or chaste tree. The vitex sports blue-violet blooms. Blue-violet and Yellow-orange just happen to be across from each other on the color wheel. Both of these blooms are "tints," which in visual art talk means that the colors have been diluted with white. The agastache is in a large pot next to the driveway, where it greets our guests and the hummingbirds that simply can't stay away from it. The vitex is in the middle of the front yard, easily fifteen feet away. But as you walk from the back of the house on the west side, this is the view.
It goes without saying (but I'll say it anyway) that it is easier to coordinate foliage with blooms than blooms with blooms, since the blooms are seasonal and subject to prevailing weather in a given year. The same goes for landscape features like stone--it is much easier to predict how the rocks will appear from season to season! So plan for the features that are less likely to change (shrubs & pathway colors), and add your perennials and annuals to fit whenever you need a complementary *Pop*!
color wheel image thanks to