title: setting NSAttributed String attribute overrides substring attributes
body: <p>I have created a mutable string  which will look like @"<strong>testMeIn:greenColor:Different:greencolor:Colors</strong>"</p>
  
  <pre><code>NSMutableAttributedString *mutableText = [[NSMutableAttributedString alloc] initWithAttributedString:myString];
  
  UIColor *foregroundColor = [UIColor blackColor];
  NSString *key = NSForegroundColorAttributeName;
  
  [mutableText addAttribute:key value:foregroundColor range:NSMakeRange(0, myString.length)];
  </code></pre>
  
  <p>When I add Attribute foregroundColor , the existing green color in substring gets overridden by the specified black color. Though I can change the code to set the green color for substring, I would like to know is there any other way of applying styles to the part of Strings which doesn't have styles without overriding the existing styles. </p>