protected function selfMaybeCompressCssColorCb(array $m)
    {
        $m['hex'] = strtoupper($m['hex']); // Convert to uppercase for easy comparison.

        if ($m['hex'][0] === $m['hex'][1] && $m['hex'][2] === $m['hex'][3] && $m['hex'][4] === $m['hex'][5]) {
            return $m['context'].$m['hex'][0].$m['hex'][2].$m['hex'][4];
        }
        return $m[0];
    }