UIColor: Is grayscale value a property?
<p><a href="https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIColor_Class/#//apple_ref/occ/clm/UIColor/blackColor" rel="nofollow">https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIColor_Class/#//apple_ref/occ/clm/UIColor/blackColor</a></p>

<blockquote>
  <p>Creating a UIColor with Preset Component Values</p>

<pre><code>+ blackColor
</code></pre>
  
  <p>Returns a color object whose grayscale value is 0.0 and whose alpha
  value is 1.0.</p>
</blockquote>

<p>Is there a method to retrieve a color objects "grayscale value"?</p>