code: public void setCornerColor(int index, Color color) {
          if (cornerColor == null) {
              for (int i = 0; i < cornerX.size(); i++) {
                  cornerColor.add(new RGBColor(this.fillColor.getRed(), this.fillColor.getGreen(),
                      this.fillColor.getBlue(), this.fillColor.getAlpha()));
              }
              setGradation(true);
          }
          if (cornerColor.size() < cornerX.size()) {
              while (cornerColor.size() != cornerX.size()) {
                  cornerColor.add(new RGBColor(this.fillColor.getRed(), this.fillColor.getGreen(),
                      this.fillColor.getBlue(), this.fillColor.getAlpha()));
              }
          }
          cornerColor.set(index, color);
      }
docs: Sets the color of a corner.
  
  @param index The index number of a corner.
  @param color The color of a corner.