Remember when choosing your external colours to go 2 to 3 shades darker in colour, as the natural sun light will fade out the colour. Depending on the look you are trying to achieve, your colour tone and intensity is important.
Make sure to buy sample pots and paint up a board with your chosen colour, take it outside to see what the colour will look like in the sun light. From here you should be able to make your decision on your colour choice.