code: private static Color[] rainbow(int n, float start, float end, float alpha) {
          return rainbow(n, start, end, 1.0f, 1.0f, alpha);
      }
docs: Generate rainbow color palette.
  @param n the number of colors in the palette.
  @param start the start of h in the HSV color model.
  @param end the start of h in the HSV color model.
  @param alpha the parameter in [0,1] for transparency.