One way to add interest to a garden is to add accent plants. These plants have a contrasting color, form, flowers, texture or size compared to the adjacent plants.
Color Accents
with Barry
Red Japanese Maple is a good contrast to the surrounding green foliage. The weeping Willow is also a good accent.
Red Ninebark (Physocarpus opulifolius ‘Lady in Red’) works well with the surrounding green plants.
Orange Rocket Barberry attracts the eye from the nearby plants.
The yellow color of the above grass-like plants make them good accents. 
Three accents in a row (Berberis ‘Orange Rocket’)
Coleus (Solenostemon scutellarioides) is a house plant that can be used as an outdoor accent for the warm season.
Blue-grey foliage of Catmint contrasts with Rubus ground cover.
Camperdown Elm’s weeping habit is a good contrast to the surrounding plants.
The weeping form of the Weeping Japanese Cherry becomes an accent even though the color and texture is similar to the background plants.
This Resteo has a strong weeping habit and attracts attention to the ponds below.
The Fairy Wand has a strong spiky accent form.
Spiky form of Dasylirion quadrangulatum
The conical form of the Alberta Spruce is a good accent. But when you have too many accents in a small area they lose their effectiveness.
The chartreuse green flower color of the Euphorbia ‘Blue Haze’ is very unusual and therefore creates an accent.
The flowers of the Saffron Buckwheat (Eriogonum umbellatum ‘Shasta Sulfur’) are a contrast to the surrounding green foliage.
Some plants are not an accent but when they are in bloom like this Fremontodendron they stand out in the landscape.
Chilean Umbrella Plant (Gunnera tinctoria)
Cardoon (Cynara cardunculus) is related to the common artichoke.
The Mexican Orange in the background becomes the accent because of its larger size compared to adjacent plants.