In certain circumstances, it can be necessary to bring exact values from either an input texture or material straight to the final pixel. Unreal’s color pipeline can make this process somewhat unclear, so this tutorial will show you one method by which you can take those values and get them directly into your final pixel using custom stencil and a postprocess material.
This technique is limited to deferred rendering and opaque/masked materials since it relies on the Base Color GBuffer.
https://dev.epicgames.com/community/learning/tutorials/LEKn/unreal-engine-using-custom-stencil-to-selectively-bypass-postprocessing