How can I extend the document structure?

To extend a document, use the add() method, to define a closure that will return an array of data to merge into the document structure:

\Aero\Search\Elastic\Documents\ListingDocument::add('search-result', function ($document) {
    $colors = $document->getModel()
        ->visibleVariants()
        ->pluck('attributes')
        ->flatten()
        ->filter(function ($attribute) {
            return strpos(strtolower($attribute->group->name), 'color') !== false;
        })
        ->pluck('name')
        ->unique()
        ->values()
        ->all();

    return [
        'colors' => $colors,
    ];
});