One of the best ways to make your garden visually interesting is to aim for contrast between your plants. This includes contrast in shape and texture, but also, and perhaps most importantly, in color. Flowers eventually fade, but plants with interesting foliage color can last through the entire growing season, or even all year as in the case of evergreens.
However, foliage coloring alone doesn't get the job done, you need to contrast plants together to really make them pop. Yellowish-green next to yellowish-green just looks like yellowish green. Put a yellowish-green plant next to a reddish-green plant though and the contrast makes both colors pop and look like brilliant yellow and vibrant red.
Plants come in various shades of green of course, but also shades of yellow (including cream), red (including burgundy or purple), and blue. Rarely you can even find plants with shades of white or silver & black, even orange. Then of course you can find variegated foliage that includes more than one color on a single leaf.
Some plants will show an interesting color only in the fall when their leaves change color, others will do so in the spring with their new growth, finally others will do so year round. Additionally some plants will show interesting foliage colors when placed in sun, but when in shade they fade, or vice versa. The reason is that sunlight and temperature affect chlorophyll. If a plant has a certain type of chlorophyll that is more dominant than the others then placing it in sun will cause that chlorophyll to take over, changing the color, this can be good or bad depending on if it's a color you like. For instance some plants lose their interesting colors and turn green when given too much sun, this is true of many variegated plants. So often in the Spring when the foliage first emerges it will be colorful, but when Summer comes and the sun becomes more intense it can fade to green.
Other plants, in contrast, need sun to make their vibrant colors come out and without it they can appear washed out. So, when picking out foliage plants, you really need to know if it needs sun or shade for utmost color.
Finally, with regards to temperature, some types of chlorophyll die at different temperatures than others, so when it gets cool in the fall the green chlorophyll in a lot of plants fades away, leaving the more colorful types. This is why many leaves change color in Autumn.
There are a few key plants often used for their striking foliage, they are as follows
Sedum, or stonecrop, are hardy perennial succulents, hardy to zone 3 usually. They come in both ground cover and upright varieties. They do have flowers, and bees particularly like their nectar, but mostly they are grown for their foliage and their hardiness. In addition to being cold tolerant they are pest, disease, and drought tolerant as well.
You can find sedum in black, purple, red, yellow, cream, green, and variegated green & yellow forms. They do need sun to bring their colors out though.
Heuchera and Heucherella
Commonly called coral bells, heuchera, and their smaller cousins heucherella, are low growing shrub-like perennials with maple-shaped leaves. Commonly found with dark burgundy coloring there are varieties that can be bought covering the entire color spectrum except blue. You can even find orange ones. Often they can be variegated as well with some interesting combinations, such as yellow and red. Wayside Gardens is a particularly good source for interestingly colored heucherellas.
Also very hardy, these plants need shade to thrive and show off their coloring, too much sun can make them more green.
Hostas are probably the most popular shade perennial for North American gardens. They come in a variety of shapes, textures, and colors, are hardy to zone 3, and really only have a pest problem with slugs.
Hostas come in greens, blues, whites, creams, and light yellows, and are often variegated. They need shade to reach their best coloring, although they can still thrive in the sun, the leaves just end up looking rather poor if they get too much.
Canna Lilies, annuals in most of the country, come in a wide range of foliage colors, have beautiful blooms, and have more complex variegation possibilities than almost any other plant.
Canna lilies can provide foliage colors in ranges from white, to yellow, to orange, to red, to burgundy, to almost black, and of course green. One cultivar, Canna 'Tropicanna' actually has leaves variegated with 5 different colors in stripes.
Canna lilies are tropical plants, so they are grown as annuals in most of the country. However you can dig them up in the fall and store them in your basement or another cool dry place and replant them in the spring. They do need a lot of water, but are also water tolerant, so many people plant them in water gardens or in pots full of water, they actually prefer and grow better in standing water. They also need full sun, as much sun as possible for good foliage coloring and so they'll bloom well.
When placing contrasting plants in your garden you will want to avoid putting them side by side in straight lines. Straight lines are for formal gardens, and contrasting plants provide an informal look. If you use contrasting plants, be they contrasting in texture, shape, or color, in a straight line, it always ends up looking funny. Instead stagger them infront of or behind each other, also try planting them in odd groups such as groups of 3. You can also plant say a group of 3 low plants of one color, say yellow, around a single taller upright plant that is red, this will look fine as a specimen.
That is it for this article, I will however write follow-ups covering plants that tend to be available in only one specific color, such as blue, red, or yellow.