code: @Override
      public Color getAtomColor(IAtom atom, Color defaultColor) {
          Color color = defaultColor;
          if (atom.getCharge() == null) return defaultColor;
          double charge = atom.getCharge();
          if (charge > 0.0) {
              if (charge < 1.0) {
                  int index = 255 - (int) (charge * 255.0);
                  color = new Color(index, index, 255);
              } else {
                  color = Color.blue;
              }
          } else if (charge < 0.0) {
              if (charge > -1.0) {
                  int index = 255 + (int) (charge * 255.0);
                  color = new Color(255, index, index);
              } else {
                  color = Color.red;
              }
          }
          return color;
      }
docs: Returns the a color reflecting the given atom's partial charge, or
  defaults to the given color if no color is defined.
  
  @param atom         IAtom to get a color for
  @param defaultColor Color returned if this scheme does not define
  a color for the passed IAtom
  @return             the color for the given atom.