How can i change background color of product image

It isn't good idea to edit core files, but the easiest way to do that is to edit /app/code/core/Mage/Catalog/Model/Product/Image.php and edit the color code at the line 44

 protected $_backgroundColor  = array(255, 255, 255);

And replace the white color code to black as follows:

 protected $_backgroundColor  = array(0, 0, 0);

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