title: "Inverse" UIImage from NSAttributedString
body: <p>I create a <code>UIImage</code> from an <code>NSAttributedString</code>:</p>
  
  <pre><code>UIFont *font = [UIFont systemFontOfSize:12.0f];
  NSDictionary *attributes = @{NSFontAttributeName:font,
                               NSForegroundColorAttributeName:[UIColor greenColor]};
  
  NSAttributedString *attributedString = [[NSAttributedString alloc] initWithString:title attributes:attributes];
  
  // Create the image here
  UIImage *image = [self imageFromAttributedString:attributedString];
  
  
  #pragma mark - Get image from attributed string
  
  - (UIImage *)imageFromAttributedString:(NSAttributedString *)text
  {
      UIGraphicsBeginImageContextWithOptions(text.size, NO, 0.0);
  
      // draw in context
      [text drawAtPoint:CGPointMake(0.0, 0.0)];
  
      // transfer image
      UIImage *image = [UIGraphicsGetImageFromCurrentImageContext() imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
      UIGraphicsEndImageContext();
  
      return image;
  }
  </code></pre>
  
  <p>My question is, what's the best way to create an image with the same text, but "reversed", i.e. [UIColor greenColor] with white font text?</p>