SceneColor is the base colour for the rendered scene, I can’t see why it would have an alpha channel.
For your effect though, it sounds like you want an individual object to turn transparent so you could see what’s behind it. That wouldn’t be a post-process effect but a material effect on the object itself.
Post-process would be used with the stencil here if you wanted to change the colour using the stencil, blur it or completely replace that section with some other colour. In this case if you wanted to replace it with a different colour you would just literally just change the scene colour if the stencil value is the one you want.