protected function randomCssColor()
    {
        if ($this->ismono) {
            $alpha = 1 - $this->rand(0, 96) / 100;
            return "rgba({$this->monocolor[0]}, {$this->monocolor[1]}, {$this->monocolor[2]}, $alpha)";
        }
        $r = $this->rand(0, 255);
        $g = $this->rand(0, 255);
        $b = $this->rand(0, 255);
        return "rgb($r, $g, $b)";
    }