Query WooCommerce Products based on Attribute

I’m trying to query WooCommerce products based on a specific attribute. For example, I set up an attribute called colour, with possible values of red or green.

I want to select all products with a colour value of “green”, so I query using the following:

$args = array(
'post_type' => 'product',
'meta_query' => array(
    array(
        'key' => 'pa_colour',
        'value' => 'green',
        'compare' => '='
    )
)
); 

query_posts($args);

I understand it that WooCommerce attributes are preceded with pa_, but any variant of the attribute name is just as unsuccessful. Where am I going wrong? Is it possible to query attributes in this way?

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