code: @Override
      protected void interpolate(double frac) {
          if (start == null) {
              starting();
          }
          Color newColor = start.interpolate(end, frac);
          if (Color.TRANSPARENT.equals(start)) {
              newColor = new Color(end.getRed(), end.getGreen(), end.getBlue(), newColor.getOpacity());
          }
          region.get().setBackground(new Background(new BackgroundFill(newColor, radii, insets)));
      }
docs: {@inheritDoc}