code: private Color mutateColour(Color colour, Random rng)
      {
          if (mutationProbability.nextValue().nextEvent(rng))
          {
              return new Color(mutateColourComponent(colour.getRed()),
                               mutateColourComponent(colour.getGreen()),
                               mutateColourComponent(colour.getBlue()),
                               mutateColourComponent(colour.getAlpha()));
          }
          else
          {
              return colour;
          }
      }
docs: Mutate the specified colour.
  @param colour The colour to mutate.
  @param rng A source of randomness.
  @return The (possibly) mutated colour.