Transparency with different colors?

Hi, I want to achieve transparency in a different way. By using standard transparent shader, the object is not opaque and the camera can see the objects behind. But for me the problem is that the color of the objects behind is blending with the transparent object. What I want to achieve is to make the color of behind objects something bizarre. For example,

The cube is purple, and the vertical quad is transparent. The transparency makes the color of the half cube pink from purple. But I want to make that area green, blue, or black etc. How can I achieve this? I think the answer will be around shaders, so I searched the web for hours but nothing so far is productive.