Custom Background: Can’t Set Color Default

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You have to pass the $defaults as the second parameter to add_theme_support() per the Codex

$defaults = array(
    'default-color'          => '000000',
    'default-image'          => '',
    'wp-head-callback'       => '_custom_background_cb',
    'admin-head-callback'    => '',
    'admin-preview-callback' => ''
);

add_theme_support( 'custom-background', $defaults);