When deciding how to plan a colour scheme, it can be helpful to refer to a chart called the 'colour wheel', which is based on the spectrum. On this circular diagram (also known as a spectral wheel), the primary colours (red, blue and yellow) are separated from each other by related secondary colours (violet, green and orange).

Whereas adjoining colours, such as violet and navy, are generally perceived to harmonise, those opposite each other, such as green and red, are 'complementary', that is to say, they provide the greatest contrast. The colour wheel is simply a useful tool to illustrate what many people instinctively perceive. If, for example, you were to plant the yellow Argyranthemum 'Jamaica Primrose' next to the deep purple-blue Salvia superba 'May Night', you would find that they made a satisfying and lively contrast, which would irresistibly draw your eye. It would, therefore, come as no surprise that the two colours are to be found directly opposite each other on the colour wheel and are therefore complementary.

Another way to look at the colours is by dividing the spectrum into two ranges - the yellows and the blues. The yellow range includes the creamy-whites, ivories, salmon-pinks, scarlet-reds, oranges and yellows. The blue range includes the bluish-whites, bluish-pinks, magenta, fuchsia, crimson-reds, blues and purples. Associations between the two ranges generally harmonise well, but mixing colours within a range may be less successful. However, there are exceptions: blues and mauves usually combine well, for example.

Pastel shades rarely clash but, if used exclusively, will create a dull garden; varying the intensity of colour within the scheme is therefore important. You can make a colour scheme more interesting by using flowers and foliage in one of the bright primary colours, toned down by neighbouring plants of the same colour mixed with black to give the browns, dark blues and purples, or with white to give pastel colours.