Why does the same apple appear to be different colours when illuminated by sunlight and when illuminated by a fluorescent lamp? A number of factors, such as differing light source, differing orientation, differing size, and differing background of the object can affect the way colours appear to us. They can make colours we would expect to perceive as identical appear different, and vice versa.

Differing light source

Have you ever had the experience of buying an apple that looked red and delicious at the grocer's shop then finding that it appeared far less tasty when you looked at it at home under fluorescent lighting? The characteristics of light from sources such as the sun, fluorescent lamps, and incandescent light bulbs differ. When looked at under these different types of light, the same apple will appear to have slightly different colours.

Differing orientation

If you look at a car, for instance, from slightly different positions, the paint colour may appear bright from one angle and dark from another. This tendency is particularly strong with colours with a transparent or metallic effect. This means that it is essential to look at colours from the same orientation in order to properly evaluate them against each other. Also, colours may be perceived differently depending on the direction from which they are lighted (the illumination position).

Differing size

Sometimes a shade of wallpaper we found attractive when looking at a colour sample can appear too gaudy once it has actually been hung on the wall. There is a tendency for larger areas of a colour to appear brighter and more vibrant than smaller patches. This is why it is often difficult to select the ideal cover for a large surface by looking at tiny samples.

Different backgrounds

If, for example,we view an apple placed against a bright background, it will appear darker than it would if placed against a dark background. This is due to the so-called "contrast effect" of colours. Let's take a look at a few of the ways the contrast effect can influence the way we see colours.

Successive contrast effect

Look at the green square for about 30 seconds, then look at the point in the centre of the square at right. You should see a red square. Red and green are complimentary colours. This phenomenon of a second colour appearing different to us after looking at another colour for a certain period of time is caused by the afterimage left behind from looking at the first colour.

Brightness contrast effect

The same shade of grey appears lighter when placed against a black background and darker when placed against a white background. In this case, the apparent brightness of the colour one is looking at is affected by the background.

Hue contrast effect

Against a red background, orange appears somewhat yellowish, and the same shade of orange appears somewhat reddish against a yellow background. This illustrates the way the appearance of a colour can be affected by the colour of the background it is placed against.

Chroma contrast effect

This next effect is caused when two colours of contrasting vividness are placed next to each other. When placed against a vivid background colour the blue square looks somewhat dull or cloudy, but it appears quite vivid when placed against a dull background. This effect occurs because the same colour will appear duller when contrasted with a vivid background, and more vivid when contrasted with a dull background.