If you want color in your garden all year around, I suggest that you think in terms of foliage colors first and then use flower colors as accents.
Why not plan around flower color? Because the flowers of perennials and woody plants don't last very long. You wait all year to see those beloved peonies or rhododendrons or daylilies open up. When they bloom, the flowers last anywhere from a few days to a couple of weeks and then that's it for the year.
You can add color to the garden for a longer period using annuals, but replacing them every year and keeping them looking good can be an expensive and high maintenance project.
To get ideas, look at gardens in your neighborhood. Here you see a lot of color and not a flower in sight!
Visit public gardens, too. This photo was taken in the Coenosium Rock Garden at South Seattle Community College. The garden features an outstanding collection of conifers and look at all that color! There's the blue Atlas cedar (Cedrus atlantica), the kelly green of the dwarf Alberta spruce (Picea g. albertiana 'Conica'), yellow green of deodar cedar (Cedrus deodora), and pines (Pinus sp.) in various shades of green.
Foliage color may be deciduous like the
purple smoke tree (Cotinus coggygria), or evergreen like the heavenly bamboo (Nandina domestica 'Moon Bay') shown above.
Look also for variegation in foliage, like this iris.
Many plants have colored edges on their leaves, like
Daphne 'Carol Mackie' or Hebe 'Red Edge.'
And don't forget grasses. Here you see Japanese forest grass (Hakonechloa macra) on the right and bronze leather leaf sedge (Carex buchananii) at the left. If you want more blue in your landscape, look for blue oat grass (Helictotrichon sempervirens) or blue fescue (Festuca glauca 'Elijah Blue').
Need more ideas? "
Consider the Leaf: Foliage in Garden Design," by Judy Glattstein goes much more in depth. She writes in a comfortable, conversational style, making it enjoyable to learn from her about how to bring lots of color with less effort into your garden.