How to see colors in Xcode?
This is an Xcode Playground.  That's important because it's the only place (with the default settings) that you can display these colors.

EDIT (June 23, 2016): After reviewing the WWDC material on new features in Xcode, images are also supported.  This is done via image literals.

This is taken from Apple's Playground Help:


  
  Open the source editor for the playground page that will contain the literal.
  Place the insertion point at the location in the code for the UIColor or NSColor.
  Choose Editor &gt; Insert Color Literal.
  A color literal is added at the insertion point, and the color picker opens.  The screenshot shows the color literal just inserted for the tintColor property of imageView.
  
  Use the color picker to choose a different color for the inserted literal. In the figure below, imageView.tintColor has been changed to Aqua, which updates the color of the resulting image.