It features interchangeable colors, blue and orange. (step 6)
The steps are in the photos. I have highlighted the pieces you need to add with each step.
Includes optional stand. (step 5)
See also: http://www.instructables.com/id/Lego-Atlas-And-P-Body/
Step 2: The Barrel.
Step 3: The Body.
Step 6: Interchanging Portal Colors.
Step 7: Done!
Here are some different views of the gun.