code: private function setShadowColor($color, $alpha, $type)
      {
          if ($color !== null) {
              $this->shadowProperties['color']['value'] = (string) $color;
          }
          if ($alpha !== null) {
              $this->shadowProperties['color']['alpha'] = $this->getTrueAlpha((int) $alpha);
          }
          if ($type !== null) {
              $this->shadowProperties['color']['type'] = (string) $type;
          }
  
          return $this;
      }
docs: Set Shadow Color.
  
  @param string $color
  @param int $alpha
  @param string $type
  
  @return GridLines