White light is what our eyes see when all the various wavelengths of the visible spectrum are combined. Each wavelength actually produces a slightly different response in the human eye, but the combination of all the colours gives us the so-called white light in which our normal daylight environment is bathed on a bright sunny day.
Since what we know as white light consists of a range of other colours, its colour varies as the elements within the mixture increase and decrease. If a little more red is added, perhaps by the setting sun, the white we see becomes slightly warmer. At midday, when the sky is clear blue, the white light we see becomes bluer and cooler. Other atmospheric effects can produce numerous subtle hues including orange, ivory, cream, yellow and blue. However the responses detected by the human eye are modified by the brain, and the result is that we see any naturally bright unfiltered light source as white. This effect is best illustrated by the "white" light emitted by domestic tungsten lamps, the colour of which is quite different from natural sunlight. Nevertheless, when we read a book in artificial light the eye still perceives a white page. The combination of the eye and brain has a sophisticated white balance facility similar to that built in to modern digital cameras.
In the age of relatively smart digital cameras, and Photoshop image manipulation, it is easy to overlook the extent to which we depend on whites appearing correct for all other colours to be rendered natural. Colour accuracy is important for visual comfort. If the whites are incorrect, our brains attempt to adjust the images displayed on computer monitors and digital cameras so that we perceive what we expect. However, the system breaks down when the error is large.
Colour temperature is a useful way to describe the whiteness of white light, particularly when comparing one light source with another. It can also be scientifically measured using a colour temperature meter.