This colour arises from very weak absorptions at the yellow-red end of the electromagnetic spectrum. The visible part of this spectrum stretches from the UV region, starting at a wavelength of about 380 nm, to the start of the infra-red region, at about 770 nm. These are the wavelengths that can stimulate the retinal cells of the eye and which give rise to the perception of colour.

Molecular Vibrations
Now, absorptions occur when a molecule can vibrate in harmony with the radiation falling on it. Thus, if a water molecule can vibrate at a frequency of about 4.28 x 1014 vibrations per second then light having this frequency, which has a wavelength of 700 nm, and which is perceived as red, will be absorbed by the molecule. And if you remove some of the red light from white light then it will appear bluish. ...