collection.php

<?php

$attributes = $productAttributes
    ->pluck('attributesValues')
    ->flatten()
    ->unique()
    ->groupBy(function (AttributeValue $av) {
        return $av->attribute->name;
    })
    ->map(function (Collection $collection) {
        return $collection->map(function (AttributeValue $av) {
           return $av->value;
        })->unique();
    })
    ->all();

dump($attributes);

// returns

array:2 [▼
  "Color" => Collection {#574 ▼
    #items: array:2 [▼
      0 => "red"
      6 => "blue"
    ]
  }
  "Size" => Collection {#565 ▼
    #items: array:3 [▼
      0 => "small"
      1 => "medium"
      2 => "large"
    ]
  }
]