$post_id = 25;
$meta_key = 'wpsnipp_post_background_color';
$meta_value = '#f00'; // red
update_post_meta( $post_id, $meta_key, $meta_value );

// ... Later in your theme or plugin

// Let's retrieve our color value
$color = get_post_meta( $post_id, $meta_key, true );