Magento2: How to get list of category's dropdown attributes?

Let's say that `$category` is the instance of the current category;

    $collection = $category->getProductCollection()->addAttributeToSelect(['manufacturer', 'color']);

    $manufacturers = [];

    $colors = [];

    foreach ($collection as $product) {

        $manufacturer = $product->getData('manufacturer');

        if ($manufacturer && !isset($manufacturers[$manufacturer])) {

             $manufacturers[$manufacturer] = $product->getAttributeText('manufacturer');

        }

        $color = $product->getData('color');

        if ($color && !isset($colors[$color])) {

             $colors[$color] = $product->getAttributeText('color');

        }

    }

after this, `$manufacutrers` and `$colors` should contain all the values you need. The key is the id, and the value is the option label.  

But this may cause performance issues if you have a large product collection.