Why do different impurities create different colors in diamonds?
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  In a yellow diamond, a few carbon atoms per million have been replaced
  by nitrogen atoms, each containing five valence electrons. The
  structure of the diamond crystal does not change significantly, but
  the extra electrons occupy a donor level.
  
  The nitrogen donor level energy in a diamond is large, peaking at
  about 4 eV. With a concentration of a few nitrogen atoms per million,
  instead of a clean "spike" donor level energy, the nitrogen donor
  level energy broadens into a band because of a number of complex
  factors, including thermal vibrations. This broadened donor energy
  band results in the difference between donor and conduction bands
  being as low as 2.2 eV. The most likely transition allows incident
  light with energy of 4 eV per photon to excite electrons from the
  donor level to the conduction band. However, it is possible for
  electrons to be excited to the conduction band with energy of 2.2 eV
  and upwards. This means that blue and violet light are absorbed from
  the full spectrum of light normally transmitted, and the resulting
  color is yellow.
  
  
  
  Unlike blue boron-doped diamonds, which conduct electricity,
  nitrogen-doped diamonds remain insulators. This is because nitrogen is
  a deep impurity: a relatively high energy (2.2 eV, as compared with
  0.4 eV for boron) is required to excite electrons from the donor energy level to the conduction band, and only a fraction of the
  available electrons are freed to carry a charge.
  
  An extremely rare green color can result from a higher nitrogen
  content of about 1 atom per 1000 atoms of carbon. At even higher
  nitrogen concentrations, the donor level broadens so that all visible
  light can be absorbed, resulting in a black color.
  
  Synthetic blue diamonds are created by adding boron as an impurity.
  Boron is trivalent, having three valence electrons; where a boron atom
  replaces a carbon atom in the diamond structure there is one fewer
  electron than usual. This missing electron, or hole, creates an
  acceptor energy level above the valence band. The boron acceptor
  energy is only 0.4 eV, so light of any energy can be absorbed during
  excitation. The boron acceptor band is broadened, and the absorption
  tapers off throughout the visible light energies, resulting in
  stronger absorption at the red end of the spectrum. At a level of one
  or a few boron atoms for every million carbon atoms, an attractive
  blue color results. Natural diamonds of this color, such as the Hope
  Diamond, are rare and highly priced.