What should I do to obtain such a display result?


I don’t know how to generate a single color image.

You can either resample the transformed moving image to the fixed image and blend them using an imaging filter (for generating a static image), or render the images using semi-transparent actors (for interactive use).

You can search in VTK examples for code pieces you can use. However, I would generally recommend to customize/extend existing VTK-based open-source medical image analysis and vísualization applications that can already do this, such as 3D Slicer or MITK, especially if you want to build an interactive GUI.