code: public void setCornerColor(int index, Color color) {
          if (!isGradation()) {
              for (int i = 0; i < 4; i++) {
                  cornerColor[i] = new RGBColor(0.0, 0.0, 0.0);
                  cornerColor[i] = this.fillColor;
              }
              setGradation(true);
          }
          cornerColor[index] = color;
      }
docs: Sets the color of a corner for gradation.
  
  @param index The index of a corner.
  @param color The color of a corner.