Unfortunately, the physically based rendering in Unreal doesn’t have good support for this.
The challenge is that you want “transparent” other than the red channel. Unreal doesn’t have per-channel transparency.
I was thinking you might be able to do something with an Additive deferred decal, but that doesn’t work, either (decals can’t be Additive.)
I’m assuming you want something like a “UV fluorescence” or “blood finding light” type effect. To do that, I think I’d try maybe rendering the object using a custom depth/stencil map, maybe offscreen, and then composite the fluorescence into the scene.