有人在我的博客留言帖了wiki上的一段话，也是这么说的。

When an electron in the system absorbs a photon of light of the right wavelength, it can be promoted to a higher energy level. (See particle in a box). Most of these electronic transitions are of a p-orbital electron to a p-antibonding orbital (π to π*), but non-bonding electrons can also be promoted (n to π*). Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region and are colorless to the human eye. With every double bond added, the system absorbs photons of longer wavelength (and lower energy), and the compound ranges from yellow to red in color. Compounds that are blue or green typically do not rely on conjugated double bonds alone.