Basically, it breaks down to 2 choices:

Harmonious (colors that are next to one another and share some value) or

Contrasting (colors that don’t)

Pondering the riot of colour (or lack of it) in our gardens is entirely personal and directly tied to our emotions. It's all about signature and expression. Planting a red border or filling your garden beds with blue and mauve gives insight into who you are.

Color schemes have the greatest impact when you plant large masses of color, rather than single plants.

When beginning to work with colour, start with just two—yellow and orange, for example and make small groupings, repeating the colours.

Pay attention to foliage, which always dominates the border. Look for patterns and colours in the leaves and echo them in nearby plants. For example, match the purple-red splotch in the centre of ‘Sunspot’ heucherella with a flower of the same purple.

Study how colour works in the natural landscape and mimic these effects in your garden. White separates colours from one another, emphasizing other plants.
Blue in the background makes a space look larger, while yellow lights up dark corners; keep in mind that red and purple are harder to see from a distance.
Don’t be afraid to combine vegetables with ornamentals to create strong contrasts.

Most colours and all their shades fall into three groups—warm, cool and neutral. So let's start with the neutral tones, which, despite their calm demeanour, are key players in all colour expression.

The neutral palette of flowers and foliage encompasses white, cream, beige, ivory, grey, brown, copper and black. Green by itself is also a neutral and provides the backdrop against which all other colours are displayed. Neutrals allow for transition between stronger hues. Neutrals can also be used to soften the effect of loud color schemes.

The most important use of neutral shades is cooling or intensifying the impact of stronger colours. For example, a full bed of blazing red impatiens may be so intense that we look away. But the red can be subdued when muted with soft cushions of grey-leafed snow in summer.

Neutral colours can also intensify deep petal shades. A patch of neutral white lilies intensifies blue delphiniums, just as the grey-blue leaves of the shrubby Arctic blue leaf willow intensify the warm pink of climbing ‘John Davis' roses.

When selecting containers for your pot-scapes consider black as a dramatic base for your colour scheme. The plants you choose are like the accessories you choose for your little black dress.  Coordinating the pot colour with your plant selection provides a cohesive and unified look. Whatever you do, use groupings for maximum impact and fun. Remember the rule of odd numbers.

Have fun with colour in your garden!

Visit King’s Flower and Garden for a wide selection of colourful annuals. Our colour blocking of annuals makes the selection of your flowers easy and convenient.

excerpts from:

http://www.canadiangardening.com/design-and-decor

and http://landscaping.about.com/od/flowersherbsgroundcover1/a/flower_photos.htm

http://gardening.about.com/od/gardendesign/a/GD_Color.htm