How to handle color of variation?

Hi WooCommerce, i trying to change variation dropdown text color depending from their stock and backorder sellection.

So i want, if variation is “out of stock”, and “allow backorder but notify” is selected into variation settings, then color to variation text to change to yellow, but dont know to do that. I have this function:

add_filter( 'woocommerce_variation_is_active', 
'yellow_variations_when_is_backorder', 10, 2 );
function yellow_variations_when_is_backorder( $grey_out, $variation ) {

if ( ! $variation->is_in_stock() && $variation->backorders_require_notification() )
$grey_out = true; // Here something need to change?

return $grey_out; // Here something need to change?
}

So wait for your instructions how to handle that $gray_out function.