The effect that a colour produces in an image depends upon its brightness, the area it occupies, and its relationship with areas of other colours. Colours that are adjacent in an image should be handled with particular care.
It is all too easy to get wrong the use of assertive primary colours such as saturated reds, blues and yellows. They are more likely to fight each other than muted colours, and tend to grab the eye and divert attention from the centre of interest. When photographing people it is safer to restrict their use to small areas of the image, and aim for good overall colour harmony. Look through the viewfinder not only at the subject but also at the background. If necessary, move around to alter the viewpoint. Distracting colour can often be excluded or hidden behind a person or object within the scene.
Colour harmony can be achieved in numerous ways but the most obvious is by restricting the colours in an image to closely related desaturated hues. This enhances appreciation of the subtle differences between very similar areas. For this reason successful images of people often consist of warm, muted colours that complement natural skin tones.
Harmony can also be achieved using contrasting colours although the result is likely to be more striking. Areas of colour of different sizes and strengths can be positioned to achieve an effective pictorial balance, but when they are adjacent they may produce a distracting vibrancy.Similarly, small areas of dominant colours can be used to balance larger areas of more subdued hues.
One other effect is worth mentioning. Short telephoto lenses such as those typically used to photograph people have limited depth of field, and adjacent background colours may blur and merge. Colours seen by the eye as separate entities consequently become more abstract. This is usually beneficial because it lifts the subject out of the clutter of background colour. The effect becomes more pronounced as the aperture gets larger and the distance between subject and background increases.