/**
  * construct new gradient
  *
  * @param \Jaguar\Color\RGBColor $start
  * @param \Jaguar\Color\RGBColor $end
  * @param inetger                $step
  *
  * @throws \InvalidArgumentException
  */
 public function __construct(RGBColor $start = null, RGBColor $end = null, $step = 0)
 {
     $this->setStartColor($start === null ? RGBColor::fromHex('#000') : $start);
     $this->setEndColor($end === null ? RGBColor::fromHex('#fff') : $end);
     $this->setStep($step);
 }