Composer-ready port of https://github.com/renasboy/php-color-difference with some more sugar.
composer require "bigwhoop/deltae":"~1"
use Bigwhoop\DeltaE\Color;
use Bigwhoop\DeltaE\DeltaE;
$deltaE = new DeltaE();
$c1 = Color::fromHex('#333333');
$c2 = Color::fromHex('#666666');
$closeness = $deltaE->cie2000($c1, $c2);
var_dump($closeness);
See LICENSE file.