lumosity_difference() public static méthode

The returned value should be bigger than 5 for best readability.
public static lumosity_difference ( string $color_1 = '#ffffff', string $color_2 = '#000000' ) : string
$color_1 string The 1st color.
$color_2 string The 2nd color.
Résultat string
 public function test_lumosity_difference()
 {
     $this->assertEquals('1', Kirki_Color::lumosity_difference('fff', '#ffffff'));
     $this->assertEquals('21', Kirki_Color::lumosity_difference('fff', '000'));
     $this->assertEquals('21', Kirki_Color::lumosity_difference('#000000', '#ffffff'));
     $this->assertEquals('5.23', Kirki_Color::lumosity_difference('#f2f2f2', '#c00'));
     $this->assertEquals('1.12', Kirki_Color::lumosity_difference('#f2f2f2', '#ffffff'));
 }
 /**
  * Calculate a color that will be readable against a specific background-color.
  *
  * @param   $value  string  the background color in question
  * @return  string  a color that is readable against this background-color.
  */
 public static function max_readability($value)
 {
     /**
      * The lumosity difference between the background color and white
      */
     $lumosity_difference_to_white = Kirki_Color::lumosity_difference($value, '#ffffff');
     /**
      * The lumosity difference between the background color and #333 (dark grey)
      */
     $lumosity_difference_to_black = Kirki_Color::lumosity_difference($value, '#333333');
     /**
      * Return whitre or #333333 depending on which lumosity difference is greater.
      */
     return $lumosity_difference_to_black > $lumosity_difference_to_white ? '#333333' : '#ffffff';
 }