This is what I have done ...
1 Create a cube.
2 Copy a face of the cube you want to have your transparent graphics on.
3 Move the face a little outward so that it does not share the same 3d space with the original face.
4 Create 2 material, a red one and a transparent one.
5 Remember to turn off cast shadow for the Transparent material.
6 Apply the red material to the cube.
7 Apply the transparent material to the copied face.
Attached is the file I have created for your reference.