If you are looking for a fast way to easily override all materials into a midtone grey. In this tutorial I going to show you a few ways.
You can use console commands to switch view modes - ‘viewmode lightingonly’ or use Deferred Rendering in Movie Render Queue. Unreal Engine Override all Material.
https://dev.epicgames.com/community/learning/tutorials/D1Od/clay-render-unreal-engine-5