It all boils down to the concept of color temperature .
Color temperature is a way of measuring the quality of a light source.
It is based on the ratio of the amount of blue light to the amount of red light, and the green light is ignored. The following table shows the color temperature of some light sources. Moreover, a new light bulb and new flash light have higher color temperature than their old and used equivalents, and an electronic flash is designed to have a color temperature comparable to that of average sunlight. Human brain can quickly adjust to different color temperatures. More precisely, our eyes, with the help from the experience we learned, see a white paper as a white paper no matter it is viewed under strong sunlight or in a warmly lighted room.