Selective absorption

A Yellow object is not really yellow. It has no intrinsic colour at all. The pigments or dyes which give rise to the sensation of yellow do so by subtracting from the light falling upon them most of the energy at the blue end of the spectrum. The light that is left, from the green through yellow and orange to red give rise to the sensation of yellow. To a physicist, the colour yellow is always considered in a pigment or dye to be minus blue.

The curve below show the amount of light reflected by a yellow colour throughout the visible spectrum. It is obtained by measuring electrically through a light sensitive cell the ratio of energy throughout the spectrum of the same light reflected from a near perfect white reflectance standard and from the yellow colour. It is a physicist's concept of colour and bears littile relationship to what we see as it does not take into account the light by which we see a colour nor the sensitivity to the colour of human eye.

All surface colours can be described in the same way. Green, for instance, which reflects light strongly in the middle region of the spectrum absorbs strongly at the two ends, so taking blue and red light from white light leaving only the green to be perceived.