Setting a global default size and color in Drupal Commerce product displays.

default-commerce-size-color-display-snippet.php

/*********************************************************************************
 * Setting a global default size and color in Drupal Commerce product displays.
 *
 * Include this in your custom module (renaming the function appropriately), 
 * it is not a full custom module on its own. 
 *********************************************************************************/

function MYMODULE_commerce_product_reference_default_delta_alter(&$delta, $products) {  
  $firstproduct = reset($products);
  
  // You can specify a particular product type below
  if ($firstproduct->type == 'product_type_machine_name') {
    foreach ($products as $key=>$product) {
      // Replace field names, COLORCODE and SIZECODE below according to your
      // site. This checks for a specific size and color and sets it as the
      // default selection in the product display.      
      if (($product->field_color['und'][0]['value'] == 'COLORCODE') &&
        ($product->field_size['und'][0]['value'] == 'SIZECODE')) {
        // Drupal Commerce returns either the array key or product id as delta so we
        // need to check if array key 0 exists, otherwise use product ID.
        if (isset($products[0])) {
          $delta = $key;
        } else {
          $delta = $product->product_id;
        }
        break;
      } 
    }
  } 
}