How do you plan a colour scheme when gardening with a wide palette of colour. The answer is to use complimentary colours that are directly opposite on the colour wheel. This give a lie to the old phrase about red and green which is about dress sense rather than gardening nous.
Other colour combinations that work well include yellow and violet or deep purple and for the adventurous blue and orange.
If you want a smashing colour scheme be bold and go for a medley of colour like these floral exhibits at Kew.