Contao\CoreBundle\DataContainer\PaletteManipulator::applyFallback PHP Method

    private function applyFallback(array &$config, array $action, $skipLegends = false)
    {
        if (is_callable($action['fallback'])) {
            $action['fallback']($config, $action, $skipLegends);
        } else {
            $this->applyFallbackPalette($config, $action);
        }
    }